Added ClangExternalASTSourceCommon, a local superclass

for all our external AST sources that lets us associate
arbitrary flags with the types we put into the AST
contexts.  Also added an API on ClangASTContext that
allows access to these flags given only an ASTContext
and a type.

Because we don't have access to RTTI, and because at
some point in the future we might encounter external
AST sources that we didn't make (so they don't subclass
ClangExternalASTSourceCommon) I added a magic number
that we check before doing anything else, so that we
can catch that problem as soon as it appears.

llvm-svn: 145748