Implement a proper local -> global type ID remapping scheme in the AST

reader. This scheme permits an AST file to be loaded with its type IDs
shifted anywhere in the type ID space. 

At present, the type indices are still allocated in the same boring
way they always have been, just by adding up the number of types in
each PCH file within the chain. However, I've done testing with this
patch by randomly sliding the base indices at load time, to ensure
that remapping is occurring as expected. I may eventually formalize
this in some testing flag, but loading multiple (non-chained) AST
files at once will eventually exercise the same code.

There is one known problem with this patch, which involves name lookup
of operator names (e.g., "x.operator int*()") in cases where multiple
PCH files in the chain. The hash function itself depends on having a
stable type ID, which doesn't happen with chained PCH and *certainly*
doesn't happen when sliding type IDs around. We'll need another
approach. I'll tackle that next.

llvm-svn: 136693