In the ASTReader, factor out the loading of (local) declaration IDs,

such that every declaration ID loaded from an AST file will go through
a central local -> global mapping function. At present, this change
does nothing, since the local -> global mapping function is the
identity function.

This is the mechanical part of the refactoring; a follow-up patch will
address a few remaining areas where it's not obvious whether we're
dealing with local or global IDs.

llvm-svn: 135711