Revert "Revert "Merge to upstream r304942.""

This reverts commit 38a0d5af7e.

Test: make checkbuild
Test: ./run_tests.py
Test: ./run_tests.py --bitness 64

test/libcxx/modules/cstdint_exports.sh.cpp

@@ -7,6 +7,10 @@
 //
 //===----------------------------------------------------------------------===//
 
+// This test fails on Windows because the underlying libc headers on Windows
+// are not modular
+// XFAIL: LIBCXX-WINDOWS-FIXME
+
 // REQUIRES: modules-support
 
 // Test that <cstdint> re-exports <stdint.h>