In the AST reader, switch name lookup within a DeclContect over to the

module DAG-based lookup scheme. This required some reshuffling, so
that each module stores its own mapping from DeclContexts to their
lexical and visible sets for those DeclContexts (rather than one big
"chain"). 

Overall, this allows simple qualified name lookup into the translation
unit to gather results from multiple modules, with the lookup results
in module B shadowing the lookup results in module A when B imports A.

Walking all of the lexical declarations in a module DAG is still a
mess; we'll end up walking the loaded module list backwards, which
works fine for chained PCH but doesn't make sense in a DAG. I'll
tackle this issue as a separate commit.

llvm-svn: 138463