[libc++] Proper fix for libc++'s modulemap after D68480

Summary:
In libc++, we normally #ifdef out header content instead of #erroring
out when the Standard in use is insufficient for the requirements of
the header.

Reviewers: EricWF

Subscribers: jkorous, dexonsmith, libcxx-commits, teemperor

Tags: #libc

Differential Revision: https://reviews.llvm.org/D75074