[Sema] Avoid repeated hash lookups (NFC) (llvm#131065)

Author: kazutakahirata

clang/lib/Sema/SemaOpenACCClause.cpp

@@ -2468,14 +2468,14 @@ bool SemaOpenACC::CheckDeclareClause(SemaOpenACC::OpenACCParsedClause &Clause) {
       // directives for a function, subroutine, program, or module.
 
       if (CurDecl) {
-        auto Itr = DeclareVarReferences.find(CurDecl);
-        if (Itr != DeclareVarReferences.end()) {
+        auto [Itr, Inserted] = DeclareVarReferences.try_emplace(CurDecl);
+        if (!Inserted) {
           Diag(VarExpr->getBeginLoc(), diag::err_acc_multiple_references)
               << Clause.getClauseKind();
           Diag(Itr->second, diag::note_acc_previous_reference);
           continue;
         } else {
-          DeclareVarReferences[CurDecl] = VarExpr->getBeginLoc();
+          Itr->second = VarExpr->getBeginLoc();
         }
       }
     }