[ms] Allow more unqualified lookup of types in dependent base classes

Summary:
In dependent contexts where we know a type name is required, such as a
new expression, we can recover by forming a DependentNameType.

This generalizes our existing compatibility hack for default arguments
for template type parameters.

Works towards parsing atlctrlw.h, which is PR26748.

Reviewers: avt77, rsmith

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D20500

llvm-svn: 270615