Fix the default libc++ header search path to be sysrooted. Radar 9639692.

The -cxx-isystem path is not prefixed with the sysroot directory, so it's
not a good way for the driver to set the system default C++ search path.
Instead, add -stdlib as a cc1 option and teach the frontend how to find the
headers.  The driver can then just pass -stdlib through to "cc1".

llvm-svn: 133547