[Clang] Restrict non fixed enum to a value outside the range of the...

[Clang] Restrict non fixed enum to a value outside the range of the enumeration values warning to context requiring a constant expression

In D131307 we allowed the diagnostic to be turned into a warning for a
transition period.

This had the side effect of triggering the warning in contexts not required to
be constant expression. This change will restrict the diagnostic to constant
expression contexts. This should reduce the fallout of this diagnostic.

Differential Revision: https://reviews.llvm.org/D131528