[API] Remove use of ClangASTContext from SBTarget

Summary:
The methods to find types in a Target aren't clang specific and are
pretty generalizable to type systems. Additionally, to support some of
the use cases in SBTarget, I've added a "GetScratchTypeSystems" method
to Target to support getting all type systems for a target we are
debugging.

Differential Revision: https://reviews.llvm.org/D64964

llvm-svn: 367480