[libc++] Add missing <stdbool.h> to the modulemap

[ldionne]

Summary

It used to be defined by the compiler, but libc++ now provides it.

[zoecarver]

You are the best! Thanks, Louis!

libcxx/include/module.modulemap.in

module stdatomic_h {
       @requires_LIBCXX_ENABLE_THREADS@
       requires cplusplus23
       header "stdatomic.h"
       export *
     }
     // FIXME: <stdalign.h> is missing.
     // <signal.h> provided by C library.
     // <stdarg.h> provided by compiler.
-    // <stdbool.h> provided by compiler.
+    module stdbool_h {
+      // <stdbool.h>'s __bool_true_false_are_defined macro requires textual inclusion.
+      textual header "stdbool.h"
+    }
     module stddef_h {
       // <stddef.h>'s __need_* macros require textual inclusion.
       textual header "stddef.h"
     }
     module stdint_h {
       header "stdint.h"
       export *
       // FIXME: This module only exists on OS X and for some reason the