[CMake] Enable libc++ for Fuchsia toolchain on Darwin

This is necessary in order to get a working C++ compiler on Darwin
since Clang expects libc++ headers to be part of the toolchain.

Differential Revision: https://reviews.llvm.org/D46075

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@330855 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed