[Flang] [OpenMP] Add parser support for THREAD_LIMIT clause on OMP TARGET directive.

[raghavendhra]

Summary

OpenMP 5.1 adds support for the THREAD_LIMIT clause for OMP TARGET directive.

This patch adds parser support for it in flang.

[kiranchandramohan]

LG.

[raghavendhra]

Rebasing

flang/test/Semantics/OpenMP/omp-device-constructs.f90

program main
 
   !ERROR: At most one DEFAULTMAP clause can appear on the TARGET directive
   !$omp target defaultmap(tofrom:scalar) defaultmap(tofrom:scalar)
   do i = 1, N
      a = 3.14
   enddo
   !$omp end target
 
+  !$omp target thread_limit(4)
+  do i = 1, N
+     a = 3.14
+  enddo
+  !$omp end target
+
+  !ERROR: At most one THREAD_LIMIT clause can appear on the TARGET directive
+  !$omp target thread_limit(4) thread_limit(8)
+  do i = 1, N
+     a = 3.14
+  enddo
+  !$omp end target
+
   !$omp teams num_teams(3) thread_limit(10) default(shared) private(i) shared(a)
   do i = 1, N
      a = 3.14
   enddo
   !$omp end teams
 
   !ERROR: At most one NUM_TEAMS clause can appear on the TEAMS directive
   !$omp teams num_teams(2) num_teams(3)

llvm/include/llvm/Frontend/OpenMP/OMP.td

let allowedClauses = [
     VersionedClause<OMPC_HasDeviceAddr, 51>,
     VersionedClause<OMPC_Reduction>,
     VersionedClause<OMPC_InReduction, 50>,
     VersionedClause<OMPC_Allocate>,
     VersionedClause<OMPC_UsesAllocators, 50>
   ];
   let allowedOnceClauses = [
     VersionedClause<OMPC_Device>,
+    VersionedClause<OMPC_ThreadLimit, 51>,
     VersionedClause<OMPC_DefaultMap>,
     VersionedClause<OMPC_NoWait>
   ];
 }
 def OMP_Teams : Directive<"teams"> {
   let allowedClauses = [
     VersionedClause<OMPC_Private>,
     VersionedClause<OMPC_FirstPrivate>,