[libc++] Reintroduce workaround for stdint re-export on Darwin

This had been removed as part of 3a0f88c4. It turns out that this
did break some code, but we never noticed because it requires including
<stdint.h> (and nothing else), and then using one of the types from
that header. It also requires running with modules enabled in a Standard
no later than C++17.

After the test refactorings in D145116, this bug would be caught
by running a CI job on macOS with modules enabled in C++17 mode
(but surprisingly not with more recent standards). This patch doesn't
add such a job because it is deemed a corner case.

Differential Revision: https://reviews.llvm.org/D145117