[Clang] Diagnose ill-formed constant expression when setting a non fixed enum...

[Clang] Diagnose ill-formed constant expression when setting a non fixed enum to a value outside the range of the enumeration values

DR2338 clarified that it was undefined behavior to set the value outside the
range of the enumerations values for an enum without a fixed underlying type.

We should diagnose this with a constant expression context.

Differential Revision: https://reviews.llvm.org/D130058