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

[sameeranjoshi]

Summary

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.

[DavidTruby]

LGTM

[clementval]

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

130

This should probably be done with TableGen but we can check that in another revision

[sameeranjoshi]

An XFAIL was resolved when rebased and fixed in the commit.

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

 
 void OmpStructureChecker::Enter(const parser::OmpEndSectionsDirective &x) {
   const auto &dir{std::get<parser::OmpSectionsDirective>(x.t)};
   ResetPartialContext(dir.source);
   switch (dir.v) {
     // 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
     break;
   }
 }
 
 void OmpStructureChecker::Enter(const parser::OpenMPDeclareSimdConstruct &x) {

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

!                         reduction-clause
   !$omp sections
   a = 0.0
   b = 1
   !$omp section
   c = 1
   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 |
 !                                    sections-clause
 
   !$omp parallel sections num_threads(4) private(b) lastprivate(d)
   a = 0.0
   !$omp section