Header-only IntelliSense should use language ID delivered with `didOpen` instead of defaulting to C++

Bug type: Language Service

Describe the bug

• OS and Version: linux 5.17.1
• VS Code Version: 1.65.2
• C/C++ Extension Version: 1.9.7
• Other extensions you installed (and if the issue persists after disabling them): dart, go, flutter, python, rust (literal names of the extensions)
• If using SSH remote, specify OS of remote machine:
• A clear and concise description of what the bug is, including information about the workspace (i.e. is the workspace a single project or multiple projects, size of the project, etc). Workspace is a project with a lot of go code and one file in c, a header file with just one line. This file is located at x/y.h. and contains void z(char *restrict q); which is valid C code. Language is set to C.

Steps to reproduce

1. Open x/y.h
2. See error

Expected behavior
No squiggly lines

Code sample and logs

• Code sample

void z(char *restrict q);

• Configurations in c_cpp_properties.json

• Logs from running C/C++: Log Diagnostics from the VS Code command palette

-------- Diagnostics - 04/04/2022, 19:07:04
Version: 1.9.7
Current Configuration:
{
    "name": "Linux",
    "includePath": [
        "${workspaceFolder}/**"
    ],
    "defines": [],
    "compilerPath": "/sbin/clang",
    "cStandard": "c17",
    "cppStandard": "c++14",
    "intelliSenseMode": "linux-clang-x64",
    "compilerArgs": [],
    "intelliSenseModeIsExplicit": false,
    "cStandardIsExplicit": false,
    "cppStandardIsExplicit": false,
    "mergeConfigurations": false,
    "compilerPathIsExplicit": false,
    "browse": {
        "path": [
            "${workspaceFolder}/**"
        ],
        "limitSymbolsToIncludedHeaders": true
    }
}
Translation Unit Mappings:
[ /.../x/y.h ]:
    /.../x/y.h
Translation Unit Configurations:
[ /.../x/y.h ]:
    Process ID: 17136
    Memory Usage: 12 MB
    Compiler Path: /sbin/clang
    Includes:
        /usr/include/c++/11.2.0
        /usr/include/c++/11.2.0/x86_64-pc-linux-gnu
        /usr/include/c++/11.2.0/backward
        /usr/lib/clang/13.0.1/include
        /usr/local/include
        /usr/include
    Standard Version: c++14
    IntelliSense Mode: linux-clang-x64
    Other Flags:
        --clang
        --clang_version=130001
        --header_only_fallback
Total Memory Usage: 12 MB

------- Workspace parsing diagnostics -------
Number of files discovered (not excluded): 26971
Number of files parsed: 3712

- Logs from [the language server logging](https://code.visualstudio.com/docs/cpp/enable-logging-cpp#_enable-logging-for-the-language-server)
loggingLevel: Debug
loggingLevel has changed to: Debug
cpptools/didChangeCppProperties
$/setTraceNotification
cpptools/getCodeActions: /.../x/y.h (id: 149)
cpptools/textEditorSelectionChange
cpptools/textEditorSelectionChange
cpptools/activeDocumentChange: /.../x/y.h
cpptools/getFoldingRanges: /.../x/y.h (id: 150)
cpptools/getCodeActions: /.../x/y.h (id: 151)
textDocument/didClose: /.../x/y.h
Database safe to open
cpptools/getCodeActions: /.../x/y.h (id: 152)
textDocument/didOpen: /.../x/y.h
Checking for syntax errors: /.../x/y.h
Queueing IntelliSense update for files in translation unit of: /.../x/y.h
cpptools/textEditorSelectionChange
cpptools/getDocumentSymbols: /.../x/y.h (id: 153)
cpptools/textEditorSelectionChange
cpptools/getSemanticTokens: /.../x/y.h (id: 154)
cpptools/getDocumentSymbols
cpptools/activeDocumentChange: /.../x/y.h
cpptools/getFoldingRanges: /.../x/y.h (id: 155)
cpptools/getCodeActions: /.../x/y.h (id: 156)
cpptools/finishUpdateSquiggles
Error squiggle count: 1
terminating child process: 18585
Update IntelliSense time (sec): 0.367
cpptools/getCodeActions: /.../x/y.h(id: 157)
cpptools/getFoldingRanges: /.../x/y.h (id: 158)
cpptools/textEditorSelectionChange
cpptools/activeDocumentChange: /.../x/y.h

Start with the textDocument/didOpen handling shown in the language-server log and the header_only_fallback configuration in the diagnostics. Verify how the delivered language ID is used for x/y.h, then reproduce the C header case and confirm that valid C syntax no longer receives C++-specific squiggles.