An enum may be considered to be a complete type if it was forward
declared. It may be declared with a fixed underlying type, or, in MSVC
compatiblity mode, with no type at all.
Previously, the code was written with special handling for fixed enums.
I generalized the code to check if the underlying integer type is known,
which should be the case when targetting the MSVC C++ ABI.
Fixes PR45409
Reid Kleckner