Merging r143807:

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r143807 | chandlerc | 2011-11-05 03:41:42 -0700 (Sat, 05 Nov 2011) | 4 lines

Compute the path properly on different platforms. Specifically, compute
it the exact same way that the Clang code computes this path.

Hopefully with this, the MSVC bots will actually come back to life.
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llvm-svn: 143936