New formula for clang-omp

Builds clang-omp (http://clang-omp.github.io), a clang with support for OpenMP pragmas.
Can this test be modified to do something more substantial than e.g. `--version` or `--help`? See `cmake.rb` for an example of an application formula with a good test and `tinyxml2.rb` for an example of a library formula with a good test. 

For this I'd suggest actually trying to compile and run some `openmp` code. Thanks!