Merging r143842:

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r143842 | chandlerc | 2011-11-05 16:29:28 -0700 (Sat, 05 Nov 2011) | 3 lines

Switch Lit to directly query the driver for the builtin inclue path.
Thanks to Peter for pointing out how easy this is to do. I'm now much
happier with this solution.
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llvm-svn: 143944