[Flang][OpenMP][MLIR] Add lowering from PFT to MLIR (FIR) for OpenMP declare...

[Flang][OpenMP][MLIR] Add lowering from PFT to MLIR (FIR) for OpenMP declare target directive in Flang

This patch adds PFT lowering for the OpenMP declare target directive
in Flang to the omp dialects declare target attribute, which currently
applies to function or global operations.

Reviewers: kiranchandramohan, skatrak, jsjodin

Differential Revision: https://reviews.llvm.org/D150329