Fix regression from Deferred Concepts with lambda in var init

As reported in GH #57945, this would crash because the decl context for
the lambda was being loaded via 'getNonClosureContext', which only gets
CODE contexts, so a global lambda was getting 'nullptr' here instead.
This patch does some work to make sure we get a valid/valuable
declcontext here instead.

Author: Erich Keane

clang/lib/Sema/SemaConcept.cpp

@@ -529,9 +529,16 @@ bool Sema::CheckFunctionConstraints(const FunctionDecl *FD,
     return false;
   }
 
-  ContextRAII SavedContext{
-      *this, cast<DeclContext>(
-                 const_cast<FunctionDecl *>(FD)->getNonClosureContext())};
+  DeclContext *CtxToSave = const_cast<FunctionDecl *>(FD);
+
+  while (isLambdaCallOperator(CtxToSave) || FD->isTransparentContext()) {
+    if (isLambdaCallOperator(CtxToSave))
+      CtxToSave = CtxToSave->getParent()->getParent();
+    else
+      CtxToSave = CtxToSave->getNonTransparentContext();
+  }
+
+  ContextRAII SavedContext{*this, CtxToSave};
   LocalInstantiationScope Scope(*this, !ForOverloadResolution ||
                                            isLambdaCallOperator(FD));
   llvm::Optional<MultiLevelTemplateArgumentList> MLTAL =

clang/test/SemaTemplate/concepts-lambda.cpp

@@ -0,0 +1,15 @@
+// RUN: %clang_cc1 -std=c++20 -verify %s
+// expected-no-diagnostics
+
+namespace GH57945 {
+  template<typename T>
+    concept c = true;
+
+  template<typename>
+    auto f = []() requires c<void> {
+    };
+
+  void g() {
+      f<int>();
+  };
+}