Don't treat a CXXScopeSpec with a nested name specifier but no location

as invalid.

We create those when forming trivial type source information with no
associated location, which, unfortunately, we do create in some cases
(when a TreeTransform with no base location is used to transform a
QualType).

This would previously lead to rejects-valid bugs when we misinterpreted
these constructs as having no nested-name-specifier.