[Concepts] Fix incorrect DeclContext for transformed RequiresExprBodyDecl

We would assign the incorrect DeclContext when transforming the RequiresExprBodyDecl, causing incorrect
handling of 'this' inside RequiresExprBodyDecls (bug #45162).

Assign the current context as the DeclContext of the transformed decl.

(cherry picked from commit 9769e1e)

Author: saarraz

clang/lib/Sema/TreeTransform.h

@@ -11303,7 +11303,7 @@ TreeTransform<Derived>::TransformRequiresExpr(RequiresExpr *E) {
       SemaRef, Sema::ExpressionEvaluationContext::Unevaluated);
 
   RequiresExprBodyDecl *Body = RequiresExprBodyDecl::Create(
-      getSema().Context, E->getBody()->getDeclContext(),
+      getSema().Context, getSema().CurContext,
       E->getBody()->getBeginLoc());
 
   Sema::ContextRAII SavedContext(getSema(), Body, /*NewThisContext*/false);

clang/test/SemaTemplate/instantiate-requires-expr.cpp

@@ -164,6 +164,19 @@ namespace expr_requirement {
   struct r3 {};
 
   using r3i = r3<int, unsigned int>; // expected-error{{constraints not satisfied for class template 'r3' [with Ts = <int, unsigned int>]}}
+
+  template<typename T>
+  struct r4 {
+      constexpr int foo() {
+        if constexpr (requires { this->invalid(); })
+          return 1;
+        else
+          return 0;
+      }
+
+      constexpr void invalid() requires false { }
+  };
+  static_assert(r4<int>{}.foo() == 0);
 }
 
 namespace nested_requirement {