Merging r143823:

------------------------------------------------------------------------
r143823 | chandlerc | 2011-11-05 13:55:50 -0700 (Sat, 05 Nov 2011) | 4 lines

Teach lit to ask the Clang it is running what version string to use
rather than presuming that it is 3.0. This is extra important as the
version should be 3.1, but CMake hasn't caught up with the times.
That'll be fixed in a separate commit.
------------------------------------------------------------------------

llvm-svn: 143938