[AST] Update DeclContext parent in Decl::setDeclContext

If the decl itself is a DeclContext, make sure to
update its parent too.

Author: hamishknight

include/swift/AST/DeclContext.h

@@ -261,7 +261,8 @@ class alignas(1 << DeclContextAlignInBits) DeclContext
   friend class Initializer; // uses setParent
   friend class AutoClosureExpr; // uses setParent
   friend class AbstractClosureExpr; // uses setParent
-  
+  friend class Decl; // uses setParent
+
   template<class A, class B, class C>
   friend struct ::llvm::CastInfo;
 

lib/AST/Decl.cpp

@@ -767,6 +767,9 @@ void Decl::setInvalid() {
 
 void Decl::setDeclContext(DeclContext *DC) { 
   Context = DC;
+  // If this Decl is also a DeclContext, we need to update its parent too.
+  if (auto *thisDC = dyn_cast<DeclContext>(this))
+    thisDC->setParent(DC);
 }
 
 bool Decl::isUserAccessible() const {