This adds explicit support for the WASI platform to libcxx. WASI libc uses some components from musl, however it's not fully compatible with musl, so we're planning to stop using _LIBCPP_HAS_MUSL_LIBC and customize for WASI libc specifically. Differential Revision: https://reviews.llvm.org/D61336 Reviewers: sbc100, ldionne llvm-svn: 359703