No longer accept scoped enumerations in C

We had a think-o that would allow a user to declare a scoped
enumeration in C language modes "as a C++11 extension". This is a
think-o because there's no way for the user to spell the name of the
enumerators; C does not have '::' for a fully-qualified name. See
commit d0d87b59 for details on why this
is unintentional for C.

Fixes #42372