[Flang][OpenMP] Implement workdistribute construct lowering (#140523)

This PR introduces a new pass "lower-workdistribute"
Fortran array statements are lowered to fir as fir.do_loop unordered.
"lower-workdistribute" pass works mainly on identifying "fir.do_loop
unordered" that is nested in target{teams{workdistribute{fir.do_loop
unordered}}} and lowers it to
target{teams{parallel{wsloop{loop_nest}}}}. It hoists all the other ops
outside target region. Relaces heap allocation on target with
omp.target_allocmem and deallocation with omp.target_freemem from host.
Also replaces runtime function "Assign" with omp.target_memcpy from
host.

This pass implements following rewrites and optimisations:

- **FissionWorkdistribute**: finds the parallelizable ops within teams
{workdistribute} region and moves them to their own
teams{workdistribute} region.
- **WorkdistributeRuntimeCallLower**: finds the FortranAAssign calls
nested in teams {workdistribute{}} and lowers it to unordered do loop if
src is scalar and dest is array. Other runtime calls are not handled
currently.
- **WorkdistributeDoLower**: finds the fir.do_loop unoredered nested in
teams {workdistribute{fir.do_loop unoredered}} and lowers it to teams
{parallel { distribute {wsloop {loop_nest}}}}.
- **TeamsWorkdistributeToSingle**: hoists all the ops inside teams
{workdistribute{}} before teams op.

The work in this PR is C-P and updated from @ivanradanov commits from
coexecute implementation:

[flang_workdistribute_iwomp_2024](https://github.com/ivanradanov/llvm-project/commits/flang_workdistribute_iwomp_2024)

Paper related to this work by @ivanradanov ["Automatic Parallelization
and OpenMP Offloadingof Fortran Array
Notation"](https://www.osti.gov/servlets/purl/[2449728](https://www.osti.gov/servlets/purl/2449728))

Author: skc7

flang/include/flang/Optimizer/OpenMP/Passes.td

@@ -93,6 +93,10 @@ def LowerWorkshare : Pass<"lower-workshare", "::mlir::ModuleOp"> {
   let summary = "Lower workshare construct";
 }
 
+def LowerWorkdistribute : Pass<"lower-workdistribute", "::mlir::ModuleOp"> {
+  let summary = "Lower workdistribute construct";
+}
+
 def GenericLoopConversionPass
     : Pass<"omp-generic-loop-conversion", "mlir::func::FuncOp"> {
   let summary = "Converts OpenMP generic `omp.loop` to semantically "

flang/lib/Optimizer/OpenMP/CMakeLists.txt

@@ -8,6 +8,7 @@ add_flang_library(FlangOpenMPTransforms
   MapsForPrivatizedSymbols.cpp
   MapInfoFinalization.cpp
   MarkDeclareTarget.cpp
+  LowerWorkdistribute.cpp
   LowerWorkshare.cpp
   LowerNontemporal.cpp
   SimdOnly.cpp