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

[ldionne]

Summary

This had been removed as part of 3a0f88c4c2c4. 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.

libcxx/include/module.modulemap.in

module depr [extern_c] {
     module stddef_h {
       // <stddef.h>'s __need_* macros require textual inclusion.
       textual header "stddef.h"
       export *
     }
     module stdint_h {
       header "stdint.h"
       export *
+      // FIXME: This module only exists on OS X and for some reason the
+      // wildcard above doesn't export it. Removing this breaks the tests
+      // for C compatibility headers in C++17 mode with modules enabled.
+      export Darwin.C.stdint
     }
     module stdio_h {
       // <stdio.h>'s __need_* macros require textual inclusion.
       textual header "stdio.h"
       export *
     }
     module stdlib_h {
       // <stdlib.h>'s __need_* macros require textual inclusion.