[PATCH] Fix name lookup within modules during template instantiation

[Doug Gregor]

  This looks *great*. It's simpler than I was expecting.

  One  concern about the caching, though: we might be forced to perform instantiation before we've seen all of the imports (say, due to a constexpr function template definition getting instantiated), which means the cache could be incomplete. I suggest invalidating based on the ASTReader's current generation.

http://llvm-reviews.chandlerc.com/D1204