Don't let EvaluationModes dictate whether an invalid base is OK

What we want to actually control this behavior is something more local
than an EvalutationMode. Please see the linked revision for more
discussion on why/etc.

This fixes PR31843.

Differential Revision: https://reviews.llvm.org/D29469


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@294800 91177308-0d34-0410-b5e6-96231b3b80d8