[Flang][OpenMP][Lower] Support PFT to MLIR lowering of loop-attached target constructs

This patch adds support for lowering target-related combined constructs from
PFT to MLIR. The lowering of OpenMP loop constructs is generalized in
preparation for later supporting different combinations of target, teams,
distribute, parallel and simd directives.

Currently enabled by this patch are the following combined constructs:
- do simd
- parallel do simd
- target parallel do simd
- target parallel do
- target simd

Depends on D155981 and D157090.

Differential Revision: https://reviews.llvm.org/D156455