Trying to submit 254476 one more time. This implement -gmodule debugging support.

It was previously reverted due to issues that showed up only on linux. I was able to reproduce these issues and fix the underlying cause.

So this is the same patch as 254476 with the following two fixes:
- Fix not trying to complete classes that don't have external sources
- Fix ClangASTSource::CompleteType() to check the decl context of types that it finds by basename to ensure we don't complete a type "S" with a type like "std::S". Before this fix ClangASTSource::CompleteType() would accept _any_ type that had a matching basename and copy it into the other type.

<rdar://problem/22992457>

llvm-svn: 254980