Fix unparse and add a test for nested loop constructs.

Author: jsjodin

flang/test/Parser/OpenMP/do-tile-size.f90

@@ -0,0 +1,29 @@
+! RUN: %flang_fc1 -fdebug-unparse -fopenmp -fopenmp-version=51 %s | FileCheck --ignore-case %s
+! RUN: %flang_fc1 -fdebug-dump-parse-tree -fopenmp -fopenmp-version=51 %s | FileCheck --check-prefix="PARSE-TREE" %s
+
+subroutine openmp_do_tiles(x)
+
+  integer, intent(inout)::x
+
+
+!CHECK: !$omp do
+!CHECK: !$omp tile sizes
+!$omp do
+!$omp  tile sizes(2)
+!CHECK: do
+  do x = 1, 100
+     call F1()
+!CHECK: end do
+  end do
+!CHECK: !$omp end tile
+!$omp end tile
+!$omp end do
+
+!PARSE-TREE:| | ExecutionPartConstruct -> ExecutableConstruct -> OpenMPConstruct -> OpenMPLoopConstruct
+!PARSE-TREE:| | | OmpBeginLoopDirective
+!PARSE-TREE:| | | OpenMPLoopConstruct
+!PARSE-TREE:| | | | OmpBeginLoopDirective
+!PARSE-TREE:| | | | | OmpLoopDirective -> llvm::omp::Directive = tile
+!PARSE-TREE:| | | | | OmpClauseList -> OmpClause -> Sizes -> Scalar -> Integer -> Expr = '2_4'
+!PARSE-TREE: | | | | DoConstruct
+END subroutine openmp_do_tiles