IDE: Always use the right DeclContext

We have a hack where we use the ParsedDecl as the DeclContext,
to handle completion during function signature parsing, which
happens before the FuncDecl has been created.

While fixing this properly would require a bigger change to the
parser and AST, for now we just check if the ParsedDecl is a
child context of CurDeclContext, and only use it then.

lib/IDE/CodeCompletion.cpp

@@ -5121,8 +5121,10 @@ void CodeCompletionCallbacksImpl::doneParsing() {
   if (ParsedDecl && !typecheckParsedDecl())
     return;
 
-  if (auto *DC = dyn_cast_or_null<DeclContext>(ParsedDecl))
-    CurDeclContext = DC;
+  if (auto *DC = dyn_cast_or_null<DeclContext>(ParsedDecl)) {
+    if (DC->isChildContextOf(CurDeclContext))
+      CurDeclContext = DC;
+  }
 
   Optional<Type> ExprType;
   ConcreteDeclRef ReferencedDecl = nullptr;