[coroutines] Fix unused warning on result of co_yield.

This patch follows up on r328602, which fixed the spurious unused
result warning for `co_await`.

llvm-svn: 328607
diff --git a/clang/lib/AST/Expr.cpp b/clang/lib/AST/Expr.cpp
index 3cb0013..57c8850 100644
--- a/clang/lib/AST/Expr.cpp
+++ b/clang/lib/AST/Expr.cpp
@@ -2050,7 +2050,8 @@
     return cast<GenericSelectionExpr>(this)->getResultExpr()->
       isUnusedResultAWarning(WarnE, Loc, R1, R2, Ctx);
   case CoawaitExprClass:
-    return cast<CoawaitExpr>(this)->getResumeExpr()->
+  case CoyieldExprClass:
+    return cast<CoroutineSuspendExpr>(this)->getResumeExpr()->
       isUnusedResultAWarning(WarnE, Loc, R1, R2, Ctx);
   case ChooseExprClass:
     return cast<ChooseExpr>(this)->getChosenSubExpr()->