Move StringBuilder to shared partition (dotnet/coreclr#10357)

* Move StringBuilder to shared partition

* Fix PR comments

* s/CoreClr/CoreCLR

* Remove unused ThreadSafeCopy overload

* Actually rename the file

Signed-off-by: dotnet-bot-corefx-mirror <dotnet-bot@microsoft.com>

Author: alexperovich

src/Common/src/CoreLib/System.Private.CoreLib.Shared.projitems

@@ -211,6 +211,7 @@
     <Compile Include="$(MSBuildThisFileDirectory)System\StringSplitOptions.cs"/>
     <Compile Include="$(MSBuildThisFileDirectory)System\SystemException.cs"/>
     <Compile Include="$(MSBuildThisFileDirectory)System\Text\Normalization.cs"/>
+    <Compile Include="$(MSBuildThisFileDirectory)System\Text\StringBuilder.cs"/>
     <Compile Include="$(MSBuildThisFileDirectory)System\ThreadAttributes.cs"/>
     <Compile Include="$(MSBuildThisFileDirectory)System\Threading\DeferredDisposableLifetime.cs"/>
     <Compile Include="$(MSBuildThisFileDirectory)System\Threading\Timeout.cs"/>