Support `make clang-test' when Clang is compiled outside of LLVM.

This may not work on build platforms that place the binaries on
special folders ($build_dir/bin/Release/) such as the VS IDE and
XCode. For fixing this it is necessary to add a lit.py configuration
option for saying where the Clang binaries are, and apply to that path
the same magit that is used with the path to the LLVM tools binary
directory. Doing this requires a bit of autoconf work.

llvm-svn: 124969