Migrate the Reflection serialization holders to the shared partition. (dotnet/coreclr...

Migrate the Reflection serialization holders to the shared partition. (dotnet/coreclr#10429)

* String => string, SR.GetResourceString() => SR.

* Copy over CoreRT files verbatim.

* Get things building and running again.

* Don't bring CoreRT's visibility sins into CoreCLR

* String => string, Object => object

* De-"m_" the field names.

* Move the files to the shared directory.

Commit migrated from https://github.com/dotnet/coreclr/commit/0b9b922e7a4197e478adb354189eab508a475410