[NFC][Clang][OpenMP] Use switch-case statement to process clauses of atomic directive

[tianshilei1992]

Summary

This patch makes the process of clauses of atomic directive more clear
and preparation for the support for atomic compare capture.

[tianshilei1992]

add missing break

[jdoerfert]

LG

[tianshilei1992]

Reopen to resolve build error reported by build bolt.

[tianshilei1992]

add all other clauses supported by atomic to fix assertion

clang/lib/Sema/SemaOpenMP.cpp

StmtResult Sema::ActOnOpenMPAtomicDirective(ArrayRef<OMPClause *> Clauses,
   // top and a single exit at the bottom.
   // The point of exit cannot be a branch out of the structured block.
   // longjmp() and throw() must not violate the entry/exit criteria.
   OpenMPClauseKind AtomicKind = OMPC_unknown;
   SourceLocation AtomicKindLoc;
   OpenMPClauseKind MemOrderKind = OMPC_unknown;
   SourceLocation MemOrderLoc;
   for (const OMPClause *C : Clauses) {
-    if (C->getClauseKind() == OMPC_read || C->getClauseKind() == OMPC_write ||
-        C->getClauseKind() == OMPC_update ||
-        C->getClauseKind() == OMPC_capture) {
+    switch (C->getClauseKind()) {
+    case OMPC_read:
+    case OMPC_write:
+    case OMPC_update:
+    case OMPC_capture: {
       if (AtomicKind != OMPC_unknown) {
         Diag(C->getBeginLoc(), diag::err_omp_atomic_several_clauses)
             << SourceRange(C->getBeginLoc(), C->getEndLoc());
         Diag(AtomicKindLoc, diag::note_omp_previous_mem_order_clause)
             << getOpenMPClauseName(AtomicKind);
       } else {
         AtomicKind = C->getClauseKind();
         AtomicKindLoc = C->getBeginLoc();
       }
+      break;
     }
-    if (C->getClauseKind() == OMPC_seq_cst ||
-        C->getClauseKind() == OMPC_acq_rel ||
-        C->getClauseKind() == OMPC_acquire ||
-        C->getClauseKind() == OMPC_release ||
-        C->getClauseKind() == OMPC_relaxed) {
+    case OMPC_seq_cst:
+    case OMPC_acq_rel:
+    case OMPC_acquire:
+    case OMPC_release:
+    case OMPC_relaxed: {
       if (MemOrderKind != OMPC_unknown) {
         Diag(C->getBeginLoc(), diag::err_omp_several_mem_order_clauses)
             << getOpenMPDirectiveName(OMPD_atomic) << 0
             << SourceRange(C->getBeginLoc(), C->getEndLoc());
         Diag(MemOrderLoc, diag::note_omp_previous_mem_order_clause)
             << getOpenMPClauseName(MemOrderKind);
       } else {
         MemOrderKind = C->getClauseKind();
         MemOrderLoc = C->getBeginLoc();
       }
+      break;
+    }
+    default:
+      llvm_unreachable("unknown clause is encountered");
     }
   }
   // OpenMP 5.0, 2.17.7 atomic Construct, Restrictions
   // If atomic-clause is read then memory-order-clause must not be acq_rel or
   // release.
   // If atomic-clause is write then memory-order-clause must not be acq_rel or
   // acquire.
   // If atomic-clause is update or not present then memory-order-clause must not