[libc++] Fix LWG3557 "static_cast expression in convertible_to has the wrong operand"

https://cplusplus.github.io/LWG/issue3557
I think the code change is unobservable, so we could just close this as
"Nothing To Do" instead; but it seems appropriate to follow the Standard's
wording here as closely as possible.

Differential Revision: https://reviews.llvm.org/D117964