In the AST reader, factor out the mapping of local type IDs to global

In the AST reader, factor out the mapping of local type IDs to global
type IDs into a single place, and make sure that all of the callers
use the appropriate functions to do the mapping. Since the mapping is
still the identity function, this is essentially a no-op.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@135733 91177308-0d34-0410-b5e6-96231b3b80d8