A couple of changes included:

A couple of changes included:
      - Make sure we have a way to disable building with the Roslyn compiler. That should only happen in the internal build.
      - Update the FeatureSpanOfT to also condition on using the latest compiler
      - Introduce an if/def in string.cs for code that uses the ref locals / ref return.
      - Make sure we only include the CryptographicException when building CoreCLR
      - Update the PInvokeMap.txt file to include the right set of APIs.
      - Remove duplicated APIs from PInvokeMap.txt

[tfs-changeset: 1636041]