fileid/offset pair, it now contains a bit discriminating between
mapped locations and file locations.  This separates the tables for
macros and files in SourceManager, and allows better separation of
concepts in the rest of the compiler.  This allows us to have *many*
macro instantiations before running out of 'addressing space'.

This is also more efficient, because testing whether something is a
macro expansion is now a bit test instead of a table lookup (which
also used to require having a srcmgr around, now it doesn't).

This is fully functional, but there are several refinements and
optimizations left.

llvm-svn: 40103