In Microsoft mode, make "Unqualified lookup into dependent bases of class...

In Microsoft mode, make "Unqualified lookup into dependent bases of class templates" works  inside default argument instantiation.

This is a little bit tricky because during default argument instantiation the CurContext points to a CXXMethodDecl but we can't use the keyword this or have an implicit member call generated.

This fixes 2 errors when parsing MFC code with clang.

llvm-svn: 144881