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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@210607 91177308-0d34-0410-b5e6-96231b3b80d8