[Clang] Fix ClassifyImplicitMemberAccess to handle cases where the access in...

[Clang] Fix ClassifyImplicitMemberAccess to handle cases where the access in an unevaluated context is not within a CXXRecordDecl or CXXMethodDecl

ClassifyImplicitMemberAccess assumes that if we are not in a static context then
the DeclContext must be a CXXRecordDecl or a CXXMethodDecl. In the case of the
unevaluated context this may not be true.

This will lead to a crash because contextClass will remain a nullptr.

Fixes: https://github.com/llvm/llvm-project/issues/37792
Fixes: https://github.com/llvm/llvm-project/issues/48405

Differential Revision: https://reviews.llvm.org/D142490