[libcxx] [test] Fix invocable tests on Windows

MSVC had a bug regarding preferring intergral conversions over
floating conversions. This is fixed in MSVC 19.28 and newer. Clang in
MSVC mode so far only mimics the old, buggy behaviour, but will
hopefully soon be fixed to comply with the new behaviour too
(see https://reviews.llvm.org/D99663).

Make the negative test to use a distinctly different type,
leaving checks for compiler specific bugs out of the libcxx test.

Differential Revision: https://reviews.llvm.org/D99641