swift234

Tweaks for the buildbot script.

/usr/local/bin isn't on PATH in the Jenkins environment, so add a variable to point at cmake. Jenkins' "emulate clean checkout" behavior also clobbers clang when it's checked out inside an llvm checkout, so change the script to expect LLVM and Clang checked out side-by-side and create a symlink to Clang from the LLVM tree.

Swift SVN r3972