Sema: Type-check didSet body before looking for usages of 'oldValue'

Looking at the unchecked body only worked because parse-time lookup
bound references to 'oldValue' to point at the right ParamDecl.

By type-checking the body first, the implementation of SE-0268 can
work when parse-time lookup is disabled.

Author: slavapestov

lib/Sema/TypeCheckDecl.cpp

@@ -1191,7 +1191,7 @@ bool SimpleDidSetRequest::evaluate(Evaluator &evaluator,
   // If we find a reference to the implicit 'oldValue' parameter, then it is
   // not a "simple" didSet because we need to fetch it.
   auto walker = OldValueFinder(param);
-  decl->getBody()->walk(walker);
+  decl->getTypecheckedBody()->walk(walker);
   auto hasOldValueRef = walker.didFindOldValueRef();
   if (!hasOldValueRef) {
     // If the body does not refer to implicit 'oldValue', it means we can