Move String.Comparison.cs to shared CoreLib partition (#17247)

Author: jkotas

src/mscorlib/System.Private.CoreLib.csproj

@@ -322,7 +322,6 @@
     <Compile Include="$(BclSourcesRoot)\System\Array.cs" />
     <Compile Include="$(BclSourcesRoot)\System\ThrowHelper.cs" />
     <Compile Include="$(BclSourcesRoot)\System\String.CoreCLR.cs" />
-    <Compile Include="$(BclSourcesRoot)\System\String.Comparison.cs" />
     <Compile Include="$(BclSourcesRoot)\System\Text\StringBuilder.CoreCLR.cs" />
     <Compile Include="$(BclSourcesRoot)\System\Text\StringBuilderCache.cs" />
     <Compile Include="$(BclSourcesRoot)\System\Exception.cs" />

src/mscorlib/shared/System.Private.CoreLib.Shared.projitems

@@ -495,6 +495,7 @@
     <Compile Include="$(MSBuildThisFileDirectory)System\SpanHelpers.Char.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\SpanHelpers.T.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\String.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)System\String.Comparison.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\String.Manipulation.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\String.Searching.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\StackOverflowException.cs" />