[patch] Look for libstdc++ install dir before /usr

[Rafael Espíndola]

Hi Chandler,

While working on making libc++ easier to use, I noticed that clang was
not finding even libstdc++ installed next to it. The problem is that
the libstdc++ I had installed was exactly the same version as the
system one.

The attached patch changes the driver to look first at the clang
install directory if it is not given a sysroot.

Not sure if there is a way to test this. We do have -ccc-install-dir,
but the patch only changes the case where sysroot is not set.

Cheers,
Rafael
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