Merging r143866:

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r143866 | chandlerc | 2011-11-06 01:21:07 -0700 (Sun, 06 Nov 2011) | 4 lines

Take a better approach to detecting and selecting multiarch include
directories. This way we stop at the first multiarch directory found on
the system. This achieves the real intended result of pruning
non-existent directories.
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llvm-svn: 143946