Use a ContinuousRangeMap to map from the global macro definition ID in

the AST reader down to the AST file + local ID, rather than walking
the PCH chain. More cleanup/generalization, although there is more
work to do for preprocessed entities. In particular, the
"preallocation" scheme for preprocessed entities is not going to work
well with late loading of PCH files, and it's likely we'll have to do
something akin to the SourceManager's negative/positive loading.

llvm-svn: 135556