[WIP] PFT rewrite-based do-concurrent parallelization

[skatrak]

This is a proof of concept on a PFT rewrite-based approach to do OpenMP-based parallelization of do concurrent Fotran loops. The main advantage of this approach over an MLIR pass-based one is that it should allow us to avoid re-implementing and sharing significant pieces of PFT to MLIR lowering between Flang lowering and the MLIR pass, potentially also making it much simpler to keep feature parity.

The current WIP replicates the PFT structure of an !$omp parallel do when encountering a do concurrent loop. It is still in very early stages and the resulting PFT cannot be lowered to MLIR yet, as it seems to be missing some symbol updates. However, it can already be tested:

! test.f90
subroutine foo()
  implicit none
  integer :: i

  do concurrent(i=1:10)
  end do

  !$omp parallel do
  do i=1,10
  end do
end subroutine

$ flang-new -fc1 -fdebug-unparse -fopenmp test.f90
SUBROUTINE foo
 IMPLICIT NONE
 INTEGER i
!$OMP PARALLEL DO
 DO i=1_4,10_4
 END DO
!$OMP PARALLEL DO
 DO i=1_4,10_4
 END DO
END SUBROUTINE

$ flang-new -fc1 -fdebug-dump-parse-tree -fopenmp test.f90
Program -> ProgramUnit -> SubroutineSubprogram
| SubroutineStmt
| | Name = 'foo'
| SpecificationPart
| | ImplicitPart -> ImplicitPartStmt -> ImplicitStmt ->
| | DeclarationConstruct -> SpecificationConstruct -> TypeDeclarationStmt
| | | DeclarationTypeSpec -> IntrinsicTypeSpec -> IntegerTypeSpec ->
| | | EntityDecl
| | | | Name = 'i'
| ExecutionPart -> Block
| | ExecutionPartConstruct -> ExecutableConstruct -> OpenMPConstruct -> OpenMPLoopConstruct
| | | OmpBeginLoopDirective
| | | | OmpLoopDirective -> llvm::omp::Directive = parallel do
| | | | OmpClauseList ->
| | | DoConstruct
| | | | NonLabelDoStmt
| | | | | LoopControl -> LoopBounds
| | | | | | Scalar -> Name = 'i'
| | | | | | Scalar -> Expr = '1_4'
| | | | | | | LiteralConstant -> IntLiteralConstant = '1'
| | | | | | Scalar -> Expr = '10_4'
| | | | | | | LiteralConstant -> IntLiteralConstant = '10'
| | | | Block
| | | | EndDoStmt ->
| | ExecutionPartConstruct -> ExecutableConstruct -> OpenMPConstruct -> OpenMPLoopConstruct
| | | OmpBeginLoopDirective
| | | | OmpLoopDirective -> llvm::omp::Directive = parallel do
| | | | OmpClauseList ->
| | | DoConstruct
| | | | NonLabelDoStmt
| | | | | LoopControl -> LoopBounds
| | | | | | Scalar -> Name = 'i'
| | | | | | Scalar -> Expr = '1_4'
| | | | | | | LiteralConstant -> IntLiteralConstant = '1'
| | | | | | Scalar -> Expr = '10_4'
| | | | | | | LiteralConstant -> IntLiteralConstant = '10'
| | | | Block
| | | | EndDoStmt ->
| EndSubroutineStmt ->