[Clang] treat fixed-underlying enum constants as the enumerated type in C23 to follow the spec (#172211)
Fixes #172118
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This patch resolves an issue where C23 fixed-underlying enum constants
were incorrectly treated as the underlying integer type instead of the
enumeration type.
According to C23 `6.7.2.2p15`:
> The enumeration member type of an enumerated type with a fixed
underlying type is the same as the type itself.