Darwin: Look for libc++ headers in include/, rather than lib/

Up until now we were expecting that when libc++ is installed alongside
clang the headers would be in lib/, which was true if the configure
build was used and false if the cmake build was.

We've now corrected the configure build to install in include/, and
with this change we'll be able to find the correct headers with both
build systems.

llvm-svn: 194834