Recover from missing typenames on template args for MSVC compatibility

While matching a non-type template argument against a known template
type parameter we now modify the AST's TemplateArgumentLoc to assume the
user wrote typename.  Under -fms-compatibility, we downgrade our
diagnostic from an error to an extwarn.

Reviewed by: rsmith

Differential Revision: http://reviews.llvm.org/D4049