[Clang] Tighten restrictions on enum out of range diagnostic to avoid constant initialization

The restrictions added in D131704 were not sufficient to avoid all non-constant
expression contexts. In particular constant initialization cases.

We need to check EvaluatingDecl to detect if the variable we are initializing is
constexpr or not.

At this point it looks like this is the remaining case affecting various projects
with this diagnostic.

Differential Revision: https://reviews.llvm.org/D131874