[OpenMP] Fix infinite loop on recursive initializers (#126269)

Summary:
If the user tried to initialize a gobal declare target variable with
itself the compiler will hang forever. Add a visited set to make sure
this stops.

Fixes: #69194

Author: jhuber6

clang/lib/Sema/SemaOpenMP.cpp

@@ -22819,8 +22819,12 @@ class GlobalDeclRefChecker final : public StmtVisitor<GlobalDeclRefChecker> {
   void declareTargetInitializer(Decl *TD) {
     A = TD->getAttr<OMPDeclareTargetDeclAttr>();
     DeclVector.push_back(cast<VarDecl>(TD));
+    llvm::SmallDenseSet<Decl *> Visited;
     while (!DeclVector.empty()) {
       VarDecl *TargetVarDecl = DeclVector.pop_back_val();
+      if (!Visited.insert(TargetVarDecl).second)
+        continue;
+
       if (TargetVarDecl->hasAttr<OMPDeclareTargetDeclAttr>() &&
           TargetVarDecl->hasInit() && TargetVarDecl->hasGlobalStorage()) {
         if (Expr *Ex = TargetVarDecl->getInit())

clang/test/OpenMP/declare_target_messages.cpp

@@ -33,6 +33,11 @@
 // RUN: %clang_cc1 %{common_opts_mac} -verify=expected,omp51,ompvar,omp45-to-51,omp5-and-51,omp5-or-later,omp5-or-later-var,omp45-to-51-var,omp45-to-51-clause,host-5-and-51,no-host5-and-51 -fopenmp %{limit} -o - %s
 // RUN: %clang_cc1 %{common_opts_mac} -verify=expected,omp52,ompvar,omp5-or-later,omp5-or-later-var %{openmp60} %{limit} -o - %s
 
+#pragma omp begin declare target
+static int gg;
+// expected-warning@+1 {{variable 'recursive' is uninitialized when used within its own initialization}}
+int recursive = recursive ^ 3 + gg;
+#pragma omp end declare target
 
 // expected-error@+1 {{unexpected OpenMP directive '#pragma omp end declare target'}}
 #pragma omp end declare target