On macOS, libc++ headers are distributed with the compiler, not
the sysroot. Without this, compiling a file that includes something
like <string> won't compile with gn-built clang without manual tweaks.

I used to do the manual tweaks, but now that other people are starting
to use this on mac, let's make it Just Work.

(This is marginally nicer than the cmake build now in that you can
just build 'clang' and it'll do the right thing.)

Differential Revision: https://reviews.llvm.org/D74247