First step to fixing a long lived layering violation: this

moves the MacroInfo pointer to a side hash table (which currently 
lives in IdentifierTable.cpp).  This removes a pointer from 
Identifier info, but doesn't shrink it, as it requires a new bit 
be added.  This strange approach with the 'hasmacro' bit is needed
to not lose preprocessor performance.

llvm-svn: 42722