[libcxx] [test] Avoid MSVC's non-Standard ABI in underlying_type.pass.cpp.

When compiled with Clang for Windows, this was emitting "enumerator value
evaluates to 4294967295, which cannot be narrowed to type 'int' [-Wc++11-narrowing]".

The test should more strenuously avoid poking this ABI deficiency (and it
already has coverage for explicitly specified underlying types).

Fixes D29140.

llvm-svn: 294159