ReSharper install errors for 2020.2

When installing the plugin into ReSharper 2020.2 (and 2020.2.1), the installer will show errors if ReSharper C++ is not installed:

```
Install completed with 6 errors.
12:48:21.764 ReSharperPlatformVs16_4159e2ec: Error resolving type JetBrains.ReSharper.Psi.Cpp.Caches.CppFileLocation from assembly “JetBrains.ReSharper.Psi.Cpp, Version=777.0.0.0, Culture=neutral, PublicKeyToken=1010a0d8d6380325”, request originating from assembly “JetBrains.ReSharper.Plugins.Unity, Version=2020.2.0.281, Culture=neutral, PublicKeyToken=0f46ab053645825e”. Could not find a referenced assembly by the TT_ASSEMBLYREF token.
12:48:21.834 ReSharperPlatformVs16_4159e2ec: Error resolving type JetBrains.ReSharper.Psi.Cpp.Caches.CppFileLocation from assembly “JetBrains.ReSharper.Psi.Cpp, Version=777.0.0.0, Culture=neutral, PublicKeyToken=1010a0d8d6380325”, request originating from assembly “JetBrains.ReSharper.Plugins.Unity, Version=2020.2.0.281, Culture=neutral, PublicKeyToken=0f46ab053645825e”. Could not find a referenced assembly by the TT_ASSEMBLYREF token.
12:48:21.835 ReSharperPlatformVs16_4159e2ec: Error resolving type JetBrains.ReSharper.Psi.Cpp.Caches.CppFileLocation from assembly “JetBrains.ReSharper.Psi.Cpp, Version=777.0.0.0, Culture=neutral, PublicKeyToken=1010a0d8d6380325”, request originating from assembly “JetBrains.ReSharper.Plugins.Unity, Version=2020.2.0.281, Culture=neutral, PublicKeyToken=0f46ab053645825e”. Could not find a referenced assembly by the TT_ASSEMBLYREF token.
12:48:21.957 ReSharperPlatformVs16_4159e2ec: Error resolving type JetBrains.ReSharper.Feature.Services.Cpp.CodeStyle.ICppCustomFormattingInfoProvider from assembly “JetBrains.ReSharper.Feature.Services.Cpp, Version=777.0.0.0, Culture=neutral, PublicKeyToken=1010a0d8d6380325”, request originating from assembly “JetBrains.ReSharper.Plugins.Unity, Version=2020.2.0.281, Culture=neutral, PublicKeyToken=0f46ab053645825e”. Could not find a referenced assembly by the TT_ASSEMBLYREF token.
12:48:21.958 ReSharperPlatformVs16_4159e2ec: Error resolving type JetBrains.ReSharper.Feature.Services.Cpp.CodeStyle.ICppCodeFormatterExtension from assembly “JetBrains.ReSharper.Feature.Services.Cpp, Version=777.0.0.0, Culture=neutral, PublicKeyToken=1010a0d8d6380325”, request originating from assembly “JetBrains.ReSharper.Plugins.Unity, Version=2020.2.0.281, Culture=neutral, PublicKeyToken=0f46ab053645825e”. Could not find a referenced assembly by the TT_ASSEMBLYREF token.
12:48:22.832 ReSharperPlatformVs16_4159e2ec: The type “JetBrains.ReSharper.Psi.ILanguageCppZone, JetBrains.ReSharper.Psi.Cpp, Version=777.0.0.0, Culture=neutral, PublicKeyToken=1010a0d8d6380325” is not a module zone because it does not derive from the JetBrains.Application.BuildScript.Application.Zones.IZone interface.
```

The HLSL support for shaders is based on ReSharper C++ and requires ReSharper C++ to be installed. This is always the case for Rider, but it is possible to install ReSharper without C++ support.

**Current workaround is to install ReSharper with C++ support.**

Licensing is no longer a problem as ReSharper C++ is included in both the ReSharper and dotUltimate licensing packs. Once installed, it's possible to disable C++, but this will also disable HLSL support in ShaderLab files.

See also [RSRP-480936](https://youtrack.jetbrains.com/issue/RSRP-480936)