[Clang] Allow downgrading to a warning the diagnostic for setting a non fixed...

[Clang] Allow downgrading to a warning the diagnostic for setting a non fixed enum to a value outside the range of the enumeration values

In D130058 we diagnose the undefined behavior of setting the value outside the
range of the enumerations values for an enum without a fixed underlying type.

Based on feedback we will provide users to the ability to downgrade this
diagnostic to a waring to allow for a transition period. We expect to turn this
diagnostic to an error in the next release.

Differential Revision: https://reviews.llvm.org/D131307