[FR] Support LLVM toolchain on Windows in prebuilt SDK

### [REQUIRED] Please fill in the following fields:

* Pre-built SDK from the [website](https://firebase.google.com/download/cpp) or open-source from this repo: pre-built from website
* Firebase C++ SDK version: 8.8.0
* Problematic Firebase Component: App, Auth
* Other Firebase Components in use: –
* Platform you are using the C++ SDK on: Windows
* Platform you are targeting: Windows

### [REQUIRED] Please describe the issue here:

Using the LLVM toolchain (specifically lld-link) instead of MSVC (link.exe) results in linker errors "relocation against symbol in discarded section: ..." from lld-link (see logs below).

Using link.exe instead of lld-link works fine, and it does not matter whether the source was built with MSVC or clang-cl.

As we have technical requirements to use lld-link in our projects and this is blocking us from using the Firebase C++ SDK.

#### Steps to reproduce with sample project:

1. Download Visual Studio sample project: [FirebaseCppTest.zip](https://github.com/firebase/firebase-cpp-sdk/files/7717600/FirebaseCppTest.zip)
2. Build app

#### Steps to reproduce manually:

1. Create new C++ project in Visual Studio
2. Select "LLVM (clang-cl)" as Platform Toolset in project settings
3. Link against at least firebase_app.lib and optionally firebase_auth.lib (linking both results in more errors of the same kind)
4. Add some minimal code to reference these projects, e.g.
```
App *app = App::Create();
if (app) {
Auth *auth = Auth::GetAuth(app);
printf("Created auth: %p", auth);
}
```
5. Build app

#### Error Logs

```
1>lld-link : error : relocation against symbol in discarded section: $LN10
1>>>> referenced by C:\Program Files (x86)\Microsoft Visual Studio\2019\Enterprise\VC\Tools\MSVC\14.29.30133\include\vector:803
1>>>> firebase_auth.lib(a9003852461049443285ecfe94112265_firebase_auth.dir_Debug_auth_request.obj):(__catch$??$_Emplace_reallocate@AEBQEAUNamespace@f_b_flatbuffers@@@?$vector@PEAUNamespace@f_b_flatbuffers@@V?$allocator@PEAUNamespace@f_b_flatbuffers@@@std@@@std@@QEAAPEAPEAUNamespace@f_b_flatbuffers@@QEAPEAU23@AEBQEAU23@@Z$0)
1>
1>lld-link : error : relocation against symbol in discarded section: $LN10
1>>>> referenced by C:\Program Files (x86)\Microsoft Visual Studio\2019\Enterprise\VC\Tools\MSVC\14.29.30133\include\vector:803
1>>>> firebase_app.lib(d94e1bff3c0e8414a6679ea91c3e103e_firebase_app.dir_Debug_variant_util.obj):(__catch$??$_Emplace_reallocate@AEBQEAUNamespace@f_b_flatbuffers@@@?$vector@PEAUNamespace@f_b_flatbuffers@@V?$allocator@PEAUNamespace@f_b_flatbuffers@@@std@@@std@@QEAAPEAPEAUNamespace@f_b_flatbuffers@@QEAPEAU23@AEBQEAU23@@Z$0)
1>
1>lld-link : error : relocation against symbol in discarded section: $LN10
1>>>> referenced by C:\Program Files (x86)\Microsoft Visual Studio\2019\Enterprise\VC\Tools\MSVC\14.29.30133\include\vector:803
1>>>> firebase_auth.lib(a9003852461049443285ecfe94112265_firebase_auth.dir_Debug_set_account_info_result.obj):(__catch$??$_Emplace_reallocate@AEBUUserInfoImpl@auth@firebase@@@?$vector@UUserInfoImpl@auth@firebase@@V?$allocator@UUserInfoImpl@auth@firebase@@@std@@@std@@QEAAPEAUUserInfoImpl@auth@firebase@@QEAU234@AEBU234@@Z$0)
1>
1>lld-link : error : relocation against symbol in discarded section: $LN36
1>>>> referenced by C:\Program Files (x86)\Microsoft Visual Studio\2019\Enterprise\VC\Tools\MSVC\14.29.30133\include\vector:1050
1>>>> firebase_app.lib(d94e1bff3c0e8414a6679ea91c3e103e_firebase_app.dir_Debug_variant_util.obj):($LN34)
1>
1>lld-link : error : relocation against symbol in discarded section: $LN9
1>>>> referenced by C:\Program Files (x86)\Microsoft Visual Studio\2019\Enterprise\VC\Tools\MSVC\14.29.30133\include\xtree:1679
1>>>> firebase_auth.lib(a9003852461049443285ecfe94112265_firebase_auth.dir_Debug_get_additional_user_info.obj):(__catch$??$_Copy_nodes@U_Copy_tag@?$_Tree@V?$_Tmap_traits@VVariant@firebase@@V12@U?$less@VVariant@firebase@@@std@@V?$allocator@U?$pair@$$CBVVariant@firebase@@V12@@std@@@4@$0A@@std@@@std@@@?$_Tree@V?$_Tmap_traits@VVariant@firebase@@V12@U?$less@VVariant@firebase@@@std@@V?$allocator@U?$pair@$$CBVVariant@firebase@@V12@@std@@@4@$0A@@std@@@std@@IEAAPEAU?$_Tree_node@U?$pair@$$CBVVariant@firebase@@V12@@std@@PEAX@1@PEAU21@0U_Copy_tag@01@@Z$0)
1>
1>lld-link : error : relocation against symbol in discarded section: $LN9
1>>>> referenced by C:\Program Files (x86)\Microsoft Visual Studio\2019\Enterprise\VC\Tools\MSVC\14.29.30133\include\xtree:1679
1>>>> firebase_auth.lib(a9003852461049443285ecfe94112265_firebase_auth.dir_Debug_auth_util.obj):(__catch$??$_Copy_nodes@U_Copy_tag@?$_Tree@V?$_Tmap_traits@VVariant@firebase@@V12@U?$less@VVariant@firebase@@@std@@V?$allocator@U?$pair@$$CBVVariant@firebase@@V12@@std@@@4@$0A@@std@@@std@@@?$_Tree@V?$_Tmap_traits@VVariant@firebase@@V12@U?$less@VVariant@firebase@@@std@@V?$allocator@U?$pair@$$CBVVariant@firebase@@V12@@std@@@4@$0A@@std@@@std@@IEAAPEAU?$_Tree_node@U?$pair@$$CBVVariant@firebase@@V12@@std@@PEAX@1@PEAU21@0U_Copy_tag@01@@Z$0)
1>
1>lld-link : error : relocation against symbol in discarded section: $LN9
1>>>> referenced by C:\Program Files (x86)\Microsoft Visual Studio\2019\Enterprise\VC\Tools\MSVC\14.29.30133\include\xtree:1679
1>>>> firebase_auth.lib(a9003852461049443285ecfe94112265_firebase_auth.dir_Debug_authentication_result.obj):(__catch$??$_Copy_nodes@U_Copy_tag@?$_Tree@V?$_Tmap_traits@VVariant@firebase@@V12@U?$less@VVariant@firebase@@@std@@V?$allocator@U?$pair@$$CBVVariant@firebase@@V12@@std@@@4@$0A@@std@@@std@@@?$_Tree@V?$_Tmap_traits@VVariant@firebase@@V12@U?$less@VVariant@firebase@@@std@@V?$allocator@U?$pair@$$CBVVariant@firebase@@V12@@std@@@4@$0A@@std@@@std@@IEAAPEAU?$_Tree_node@U?$pair@$$CBVVariant@firebase@@V12@@std@@PEAX@1@PEAU21@0U_Copy_tag@01@@Z$0)
1>
1>lld-link : error : relocation against symbol in discarded section: $LN10
1>>>> referenced by C:\Program Files (x86)\Microsoft Visual Studio\2019\Enterprise\VC\Tools\MSVC\14.29.30133\include\vector:803
1>>>> firebase_auth.lib(a9003852461049443285ecfe94112265_firebase_auth.dir_Debug_set_account_info_request.obj):(__catch$??$_Emplace_reallocate@V?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@@?$vector@V?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@V?$allocator@V?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@@2@@std@@QEAAPEAV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@1@QEAV21@$$QEAV21@@Z$0)
1>
1>lld-link : error : relocation against symbol in discarded section: $LN10
1>>>> referenced by C:\Program Files (x86)\Microsoft Visual Studio\2019\Enterprise\VC\Tools\MSVC\14.29.30133\include\vector:803
1>>>> firebase_auth.lib(a9003852461049443285ecfe94112265_firebase_auth.dir_Debug_get_account_info_result.obj):(__catch$??$_Emplace_reallocate@AEBUUserInfoImpl@auth@firebase@@@?$vector@UUserInfoImpl@auth@firebase@@V?$allocator@UUserInfoImpl@auth@firebase@@@std@@@std@@QEAAPEAUUserInfoImpl@auth@firebase@@QEAU234@AEBU234@@Z$0)
1>
1>lld-link : error : relocation against symbol in discarded section: $LN10
1>>>> referenced by C:\Program Files (x86)\Microsoft Visual Studio\2019\Enterprise\VC\Tools\MSVC\14.29.30133\include\vector:803
1>>>> firebase_app.lib(d94e1bff3c0e8414a6679ea91c3e103e_firebase_app.dir_Debug_variant_util.obj):(__catch$??$_Emplace_reallocate@AEBUValue@Builder@f_b_flexbuffers@@@?$vector@UValue@Builder@f_b_flexbuffers@@V?$allocator@UValue@Builder@f_b_flexbuffers@@@std@@@std@@QEAAPEAUValue@Builder@f_b_flexbuffers@@QEAU234@AEBU234@@Z$0)
1>
1>lld-link : error : relocation against symbol in discarded section: $LN10
1>>>> referenced by C:\Program Files (x86)\Microsoft Visual Studio\2019\Enterprise\VC\Tools\MSVC\14.29.30133\include\vector:803
1>>>> firebase_app.lib(d7017cfa2b4147331b16a451ec58c76c_firebase_rest_lib.dir_Debug_response.obj):(__catch$??$_Emplace_reallocate@AEBV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@@?$vector@V?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@V?$allocator@V?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@@2@@std@@QEAAPEAV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@1@QEAV21@AEBV21@@Z$0)
1>
1>lld-link : error : relocation against symbol in discarded section: $LN9
1>>>> referenced by C:\Program Files (x86)\Microsoft Visual Studio\2019\Enterprise\VC\Tools\MSVC\14.29.30133\include\xtree:1679
1>>>> firebase_app.lib(d94e1bff3c0e8414a6679ea91c3e103e_firebase_app.dir_Debug_variant.obj):(__catch$??$_Copy_nodes@U_Copy_tag@?$_Tree@V?$_Tmap_traits@VVariant@firebase@@V12@U?$less@VVariant@firebase@@@std@@V?$allocator@U?$pair@$$CBVVariant@firebase@@V12@@std@@@4@$0A@@std@@@std@@@?$_Tree@V?$_Tmap_traits@VVariant@firebase@@V12@U?$less@VVariant@firebase@@@std@@V?$allocator@U?$pair@$$CBVVariant@firebase@@V12@@std@@@4@$0A@@std@@@std@@IEAAPEAU?$_Tree_node@U?$pair@$$CBVVariant@firebase@@V12@@std@@PEAX@1@PEAU21@0U_Copy_tag@01@@Z$0)
1>
1>lld-link : error : relocation against symbol in discarded section: $LN10
1>>>> referenced by C:\Program Files (x86)\Microsoft Visual Studio\2019\Enterprise\VC\Tools\MSVC\14.29.30133\include\vector:803
1>>>> firebase_auth.lib(a9003852461049443285ecfe94112265_firebase_auth.dir_Debug_sign_in_flow.obj):(__catch$??$_Emplace_reallocate@AEBQEAUNamespace@f_b_flatbuffers@@@?$vector@PEAUNamespace@f_b_flatbuffers@@V?$allocator@PEAUNamespace@f_b_flatbuffers@@@std@@@std@@QEAAPEAPEAUNamespace@f_b_flatbuffers@@QEAPEAU23@AEBQEAU23@@Z$0)
1>
1>lld-link : error : relocation against symbol in discarded section: $LN10
1>>>> referenced by C:\Program Files (x86)\Microsoft Visual Studio\2019\Enterprise\VC\Tools\MSVC\14.29.30133\include\vector:803
1>>>> firebase_app.lib(d94e1bff3c0e8414a6679ea91c3e103e_firebase_app.dir_Debug_variant_util.obj):(__catch$??$_Emplace_reallocate@E@?$vector@EV?$allocator@E@std@@@std@@QEAAPEAEQEAE$$QEAE@Z$0)
1>
1>lld-link : error : relocation against symbol in discarded section: $LN10
1>>>> referenced by C:\Program Files (x86)\Microsoft Visual Studio\2019\Enterprise\VC\Tools\MSVC\14.29.30133\include\vector:803
1>>>> firebase_app.lib(d94e1bff3c0e8414a6679ea91c3e103e_firebase_app.dir_Debug_variant_util.obj):(__catch$??$_Emplace_reallocate@UValue@Builder@f_b_flexbuffers@@@?$vector@UValue@Builder@f_b_flexbuffers@@V?$allocator@UValue@Builder@f_b_flexbuffers@@@std@@@std@@QEAAPEAUValue@Builder@f_b_flexbuffers@@QEAU234@$$QEAU234@@Z$0)
1>
1>lld-link : error : relocation against symbol in discarded section: $LN9
1>>>> referenced by C:\Program Files (x86)\Microsoft Visual Studio\2019\Enterprise\VC\Tools\MSVC\14.29.30133\include\xtree:1679
1>>>> firebase_auth.lib(a9003852461049443285ecfe94112265_firebase_auth.dir_Debug_auth_desktop.obj):(__catch$??$_Copy_nodes@U_Copy_tag@?$_Tree@V?$_Tmap_traits@VVariant@firebase@@V12@U?$less@VVariant@firebase@@@std@@V?$allocator@U?$pair@$$CBVVariant@firebase@@V12@@std@@@4@$0A@@std@@@std@@@?$_Tree@V?$_Tmap_traits@VVariant@firebase@@V12@U?$less@VVariant@firebase@@@std@@V?$allocator@U?$pair@$$CBVVariant@firebase@@V12@@std@@@4@$0A@@std@@@std@@IEAAPEAU?$_Tree_node@U?$pair@$$CBVVariant@firebase@@V12@@std@@PEAX@1@PEAU21@0U_Copy_tag@01@@Z$0)
1>
1>lld-link : error : relocation against symbol in discarded section: $LN8
1>>>> referenced by C:\Program Files (x86)\Microsoft Visual Studio\2019\Enterprise\VC\Tools\MSVC\14.29.30133\include\vector:1252
1>>>> firebase_auth.lib(a9003852461049443285ecfe94112265_firebase_auth.dir_Debug_auth_desktop.obj):(__catch$??$_Resize_reallocate@U_Value_init_tag@std@@@?$vector@V?$unique_ptr@UUserT@fbs@auth@firebase@@U?$default_delete@UUserT@fbs@auth@firebase@@@std@@@std@@V?$allocator@V?$unique_ptr@UUserT@fbs@auth@firebase@@U?$default_delete@UUserT@fbs@auth@firebase@@@std@@@std@@@2@@std@@AEAAX_KAEBU_Value_init_tag@1@@Z$0)
1>
1>lld-link : error : relocation against symbol in discarded section: $LN28
1>>>> referenced by C:\Program Files (x86)\Microsoft Visual Studio\2019\Enterprise\VC\Tools\MSVC\14.29.30133\include\vector:993
1>>>> firebase_app.lib(d94e1bff3c0e8414a6679ea91c3e103e_firebase_app.dir_Debug_variant_util.obj):(__catch$??$_Insert_range@PEBE@?$vector@EV?$allocator@E@std@@@std@@AEAAXV?$_Vector_const_iterator@V?$_Vector_val@U?$_Simple_types@E@std@@@std@@@1@PEBE1Uforward_iterator_tag@1@@Z$0)
1>
1>lld-link : error : relocation against symbol in discarded section: $LN10
1>>>> referenced by C:\Program Files (x86)\Microsoft Visual Studio\2019\Enterprise\VC\Tools\MSVC\14.29.30133\include\vector:803
1>>>> firebase_auth.lib(a9003852461049443285ecfe94112265_firebase_auth.dir_Debug_user_desktop.obj):(__catch$??$_Emplace_reallocate@AEBQEAUNamespace@f_b_flatbuffers@@@?$vector@PEAUNamespace@f_b_flatbuffers@@V?$allocator@PEAUNamespace@f_b_flatbuffers@@@std@@@std@@QEAAPEAPEAUNamespace@f_b_flatbuffers@@QEAPEAU23@AEBQEAU23@@Z$0)
1>
1>lld-link : error : relocation against symbol in discarded section: $LN32
1>>>> referenced by C:\Program Files (x86)\Microsoft Visual Studio\2019\Enterprise\VC\Tools\MSVC\14.29.30133\include\vector:1026
1>>>> firebase_app.lib(d94e1bff3c0e8414a6679ea91c3e103e_firebase_app.dir_Debug_variant_util.obj):($LN30)
1>
1>lld-link : error : too many errors emitted, stopping now (use /errorlimit:0 to see all errors)
```