[Clang] Tighten restrictions on enum out of range diagnostic

In D131528 using Info.EvalMode == EvalInfo::EM_ConstantExpression is not strict
enough to restrict the diagnostic to only constant expression contexts. It is
sometimes set in cases where we are still determining if we are in a constant
expression context.

Using InConstantContext will tighten the restriction.

Differential Revision: https://reviews.llvm.org/D131704