Merging r143752:

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r143752 | chandlerc | 2011-11-04 16:49:05 -0700 (Fri, 04 Nov 2011) | 5 lines

Begin the migration of header search logic to the driver, starting with
Windows. There are still FIXMEs and lots of problems with this code.
Some of them will be addressed shortly by my follow-up patches, but most
are going to wait until we isolate this code and can fix it properly.
This version should be no worse than what we had before.
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llvm-svn: 143930