[mlir][OpenMP]Support for modifiers in workshare loops

Pass the modifiers from the Flang parser to FIR/MLIR workshare
loop operation.

Not yet supporting the SIMD modifier, which is a bit more work
than just adding it to the list of modifiers, so will go in a
separate patch.

This adds a new field to the WsLoopOp.

Also add test for dynamic WSLoop, checking that dynamic schedule calls
the init and next functions as expected.

Reviewed By: ftynse

Differential Revision: https://reviews.llvm.org/D111053