[Flang][OpenMP][MLIR] Add early outlining pass for omp.target operations to flang

This patch implements an early outlining transform of omp.target operations in
flang. The pass is needed because optimizations may cross target op region
boundaries, but with the outlining the resulting functions only contain a
single omp.target op plus a func.return, so there should not be any opportunity
to optimize across region boundaries.

The patch also adds an interface to be able to store and retrieve the parent
function name of the original target operation. This is needed to be able to
create correct kernel function names when lowering to LLVM-IR.

Reviewed By: kiranchandramohan, domada

Differential Revision: https://reviews.llvm.org/D154879