Change magic string "abc" to better magic string "qux".

Wait, what?

So, we run Clang (and LLVM) tests in an environment where the md5sum of the
input files becomes a component of the path. When testing the preprocessor,
the path becomes part of the output (in line directives). In this test, we
were grepping for the absence of "abc" in the output. When the stars aligned
properly, the md5sum component of the path contained "abc" and the test
failed. Oops.

llvm-svn: 131147