[CSDiag] NFC: Move findResolvedMemberRef to ConstraintSystem.cpp

This method is used by diagnostics and CSApply at this point.

lib/Sema/ConstraintSystem.cpp

@@ -3940,3 +3940,19 @@ Expr *ConstraintSystem::buildAutoClosureExpr(Expr *expr,
   cacheExprTypes(result);
   return result;
 }
+
+/// If an UnresolvedDotExpr, SubscriptMember, etc has been resolved by the
+/// constraint system, return the decl that it references.
+ValueDecl *ConstraintSystem::findResolvedMemberRef(ConstraintLocator *locator) {
+  // See if we have a resolution for this member.
+  auto overload = findSelectedOverloadFor(locator);
+  if (!overload)
+    return nullptr;
+
+  // We only want to handle the simplest decl binding.
+  auto choice = overload->choice;
+  if (choice.getKind() != OverloadChoiceKind::Decl)
+    return nullptr;
+
+  return choice.getDecl();
+}