Disallow _BitInt as an underlying type for an enumeration

As mentioned in #69619, C23 6.7.2.2p5 explicitly prohibits using a
_BitInt as an underlying type to an enumeration. While we had this in
the _ExtInt implementation, the justification for that limitation in C
is compelling, so this is being removed to be compatible with the C23
standard.

Fixes: #69619