[SemaCXX] Set promotion type for enum if its type is promotable to integer...

[SemaCXX] Set promotion type for enum if its type is promotable to integer type even if it has no definition.

EnumDecl's promotion type is set either to the parsed type or calculated type
after completing its definition. When it's bool type and has no definition,
its promotion type is bool which is not allowed by clang.

Fixes #56560.

Differential Revision: https://reviews.llvm.org/D130210