[OpenMP] Ensure the DefaultMapperId has a location

A user reported an assertion (below) but without a reproducer. I failed to
create a test myself but from the assertion one can derive the problem.
I set the DefaultMapperId location now to make sure this doesn't cause
trouble.

```
clang-13: .../DeclTemplate.h:1940:
void clang::ClassTemplateSpecializationDecl::setPointOfInstantiation(clang::SourceLocation):
Assertion `Loc.isValid() && "point of instantiation must be valid!"' failed.
```

Reviewed By: JonChesterfield

Differential Revision: https://reviews.llvm.org/D100621