[HIP][OpenMP] Fix assertion in deferred diag

Fix assertion in UsedDeclVisitor where clang is trying to look up a destructor
for a forward declared class.

Fixes: https://bugs.llvm.org/show_bug.cgi?id=52250

Reviewed by: Artem Belevich, John McCall

Differential Revision: https://reviews.llvm.org/D112235