[Clang] Improve diagnostics when using a concept as template argument

When using the name of a template variable or concept in places
where an expression was expected, Clang would drop the cxxscope token
preceeding it, if any.

This leads to subpar diagnostics - complaining about the
identifier being undeclared as clang would not know to look into a
non-global scope.

We make sure the scope is preserved.

When encountering `ns::Concept foo x;`, Clang would also fail
to provide the same quality as it does at global scope.

Reviewed By: aaron.ballman, erichkeane

Differential Revision: https://reviews.llvm.org/D146719