[MLIR][OpenMP][OMPIRBuilder] Support omp.target 'if' translation to LLVM IR

[skatrak]

This patch adds missing MLIR to LLVM IR translation support for the if clause on omp.target operations. This completes the missing piece for Fortran support of !$omp target if(...).

The implementation updates emitTargetCall() in the OMPIRBuilder to follow clang's support for the if clause in CGOpenMPRuntime::emitTargetCall().

[mjklemm]

LGTM, but I have had a hard time following the changes as the diff was sort of messed up. So, second pair of eyes might be useful.

[skatrak]

LGTM, but I have had a hard time following the changes as the diff was sort of messed up. So, second pair of eyes might be useful.

Yes, it flags a bunch of changes because I basically extracted most of the body of emitTargetCall into a couple of lambdas. The only real change is at the bottom of the function, where it's decided when to run each part. It's generally a simple change: just adding an if-else depending on the runtime value of the if expression, if present, otherwise it does the same thing it used to.

[dpalermo]

Verified this changes works correctly on Leopold's reproducer.

[bhandarkar-pranav]

@skatrak thank you for the PR. LGTM.