[Sema] Fix SR-1681: spurious "unused result" warning in Swift 3

An OptionalEvaluationExpr can have a CallExpr directly as its SubExpr,
if the CallExpr already has optional type.

extends ea52d4f

Author: timbodeit

lib/Sema/TypeCheckStmt.cpp

@@ -1080,9 +1080,12 @@ void TypeChecker::checkIgnoredExpr(Expr *E) {
   // If we have an OptionalEvaluationExpr at the top level, then someone is
   // "optional chaining" and ignoring the result.  Produce a diagnostic if it
   // doesn't make sense to ignore it.
-  if (auto *OEE = dyn_cast<OptionalEvaluationExpr>(valueE))
+  if (auto *OEE = dyn_cast<OptionalEvaluationExpr>(valueE)) {
     if (auto *IIO = dyn_cast<InjectIntoOptionalExpr>(OEE->getSubExpr()))
       return checkIgnoredExpr(IIO->getSubExpr());
+    if (auto *C = dyn_cast<CallExpr>(OEE->getSubExpr()))
+      return checkIgnoredExpr(C);
+  }
 
   // Check if we have a call to a function not marked with
   // '@discardableResult'.