Teach Sema::ActOnCXXNestedNameSpecifier and Sema::CheckTemplateIdType

Teach Sema::ActOnCXXNestedNameSpecifier and Sema::CheckTemplateIdType
to cope with non-type templates by providing appropriate
errors. Previously, we would either assert, crash, or silently build a
dependent type when we shouldn't. Fixes PR9226.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@127037 91177308-0d34-0410-b5e6-96231b3b80d8