When cloning the clang instance in ClangImporter to perform tasks like
PCM/PCH compilation, make two compiler instance to share the same
CASOptions so they can share the CAS ObjectStore and life time.
This fixes a case where clang emits a diagnostics in a source buffer that
gets mapped in via CAS, and mapped into swift source manager. While CAS has
the ownship from CompilerInvocation -> CASOptions -> CAS, if the
CompilerInvocation is deleted when the cloned instance is deleted, it
left an invalid buffer in the swift source manager.
rdar://141284501
