patch: more builtin support

Looks great to me with one minor change: we don't want the clang
testsuite to actually execute code. Please make the test just be
something like this: ...

Done. The grep works in this case but this policy is going to be harder
to deal with for other codegen tests -- either the test is going to be very
fragile or just not written.

To take a current example, how do you check that the accesses to the
following bit-field are being compiled correctly without running code:

I agree that this doesn't make sense to grep for, I just think execution tests should go in the llvm-test suite (where llvm-gcc will also get them), e.g. in the test/SingleSource/UnitTests directory.