Improve anonymous class heuristic in ClangASTContext::CreateRecordType

Summary:
Currently the heuristic used in ClangASTContext::CreateRecordType to identify an anonymous class is that there is that name is a nullptr or simply a null terminator. This heuristic is not accurate since it will also sweep up unnamed classes and lambdas. The improved heuristic relies on the requirement that an anonymous class must be contained within a class.

Differential Revision: https://reviews.llvm.org/D66175

llvm-svn: 368937