LLVM front-end tests

Hello everyone,

We have a set of tests in the llvm/test/Frontend* directories which
depend on llvm-gcc.
I have randomly checked some tests there, and it seems they should
work well with clang as well.

Shell we generalize and use them with either llvm-gcc or clang
front-end (whichever is a part of the current build or avalable), or
keep them as llvm-gcc tests?



Generalizing them to work with either would be ideal, but may be
a lot of work for marginal gain. I know we've thought about it in the past.