Add support for '33'-suffixed Clang/LLVM commands.

When using the clang33 port on FreeBSD, we need to look for commands like 'FileCheck33', 'llvm-config33' and 'clang33'. Some extra CMake flags should be set:

  -D CMAKE_LLVM_CONFIG=llvm-config33
  -D CMAKE_LLVM_LIT=llvm-lit33

This lets us use existing FreeBSD packages for Clang/LLVM 3.3 rather than having to rebuild from source.