Make it easy to include NetCore-only and NetFx-only files (#10572)

This migrates a change from Razor tooling and makes it available for all
the Razor repo. Essentially, this allows a C# file to be compiled for
just .NET Framework or .NET Core by its name or the name of the folder
its contained in.

1. Naming a C# file as `.NetCore.cs` or `.NetFx.cs` it will compile
_only_ for NetCore or NetFx, respectively.
2. C# files placed in a folder called `NetCore` or `NetFx` will compile
_only_ for NetCore or NetFx, respectively.
3. C# files placed in a folder whose name ends in `_NetCore` or `_NetFx`
will compile _only_ for NetCore or NetFx, respectively.
 
This makes it clear from the Solution Explorer if there are files that
only compile for one target or another without having to open them and
look at `#if..#endif` blocks.

Directory.Build.targets

@@ -34,6 +34,36 @@
     <SupportedPlatform Remove="iOS" />
   </ItemGroup>
 
+  <!--
+    We provide a simple mechanism for including NetFx-only files.
+    If a C# file should only be compiled under NetFx, it can be named with a ".NetFx.cs" extension.
+    Or, the file can be included in a folder called "NetFx" or has a name that ends in "_NetFx".
+  -->
+  <ItemGroup Condition="'$(TargetFramework)' != '$(DefaultNetFxTargetFramework)'">
+    <Compile Remove="**\*.NetFx.cs" />
+    <Compile Remove="**\NetFx\**\*.*" />
+    <Compile Remove="**\*_NetFx\**\*.*" />
+
+    <None Include="**\*.NetFx.cs" />
+    <None Include="**\NetFx\**\*.*" />
+    <None Include="**\*_NetFx\**\*.*" />
+  </ItemGroup>
+
+  <!--
+    We provide a simple mechanism for including NetCore-only files.
+    If a C# file should only be compiled under NetCore, it can be named with a ".NetCore.cs" extension.
+    Or, the file can be included in a folder called "NetCore" or has a name that ends in "_NetCore".
+  -->
+  <ItemGroup Condition="'$(TargetFramework)' != '$(NetCurrent)' AND '$(TargetFramework)' != '$(NetPrevious)'">
+    <Compile Remove="**\*.NetCore.cs" />
+    <Compile Remove="**\NetCore\**\*.*" />
+    <Compile Remove="**\*_NetCore\**\*.*" />
+
+    <None Include="**\*.NetCore.cs" />
+    <None Include="**\NetCore\**\*.*" />
+    <None Include="**\*_NetCore\**\*.*" />
+  </ItemGroup>
+
   <!--
     Warn consumers until the underlying issue is fixed
     https://github.com/dotnet/roslyn/issues/72657