[Flang][OpenMP] Fix issue in only a single nowait clause can appear on a sections directive.

The OpenMP 5.0 standard restricts nowait clause to appear only once on sections
directive.
See OpenMP 5.0
  - 2.8.1
  - point 3 in restrictions.

Added a test with fix.

Reviewed By: DavidTruby

Differential Revision: https://reviews.llvm.org/D88556

flang/lib/Semantics/check-omp-structure.cpp

@@ -127,7 +127,7 @@ void OmpStructureChecker::Enter(const parser::OmpEndSectionsDirective &x) {
     // 2.7.2 end-sections -> END SECTIONS [nowait-clause]
   case llvm::omp::Directive::OMPD_sections:
     SetContextDirectiveEnum(llvm::omp::Directive::OMPD_end_sections);
-    SetContextAllowed(OmpClauseSet{llvm::omp::Clause::OMPC_nowait});
+    SetContextAllowedOnce(OmpClauseSet{llvm::omp::Clause::OMPC_nowait});
     break;
   default:
     // no clauses are allowed

flang/test/Semantics/omp-clause-validity01.f90

@@ -261,6 +261,17 @@ use omp_lib
   d = 2
   !ERROR: NUM_THREADS clause is not allowed on the END SECTIONS directive
   !$omp end sections num_threads(4)
+
+  !$omp parallel
+  !$omp sections
+    b = 1
+  !$omp section
+    c = 1
+    d = 2
+  !ERROR: At most one NOWAIT clause can appear on the END SECTIONS directive
+  !$omp end sections nowait nowait
+  !$omp end parallel
+
   !$omp end parallel
 
 ! 2.11.2 parallel-sections-clause -> parallel-clause |

flang/test/Semantics/omp-sections01.f90

@@ -1,5 +1,4 @@
 ! RUN: %S/test_errors.sh %s %t %f18 -fopenmp
-! XFAIL: *
 
 ! OpenMP Version 4.5
 ! 2.7.2 sections Construct
@@ -10,7 +9,7 @@ program omp_sections
   !$omp sections
     !$omp section
     print *, "omp section"
-  !ERROR: Only a single nowait clause can appear on a sections directive.
+  !ERROR: At most one NOWAIT clause can appear on the END SECTIONS directive
   !$omp end sections nowait nowait
 
 end program omp_sections