[coroutines] Add co_return statement emission

Summary:
Added co_return statement emission.

Tweaked coro-alloc.cpp test to use co_return to trigger coroutine processing instead of co_await, since this change starts emitting the body of the coroutine and await expression handling has not been upstreamed yet.

Reviewers: rsmith, majnemer, EricWF, aaron.ballman

Reviewed By: rsmith

Subscribers: majnemer, llvm-commits, mehdi_amini

Differential Revision: https://reviews.llvm.org/D29979

llvm-svn: 297076
diff --git a/clang/lib/CodeGen/CodeGenFunction.h b/clang/lib/CodeGen/CodeGenFunction.h
index 347908b..659ec0f 100644
--- a/clang/lib/CodeGen/CodeGenFunction.h
+++ b/clang/lib/CodeGen/CodeGenFunction.h
@@ -2506,6 +2506,7 @@
   void EmitObjCAutoreleasePoolStmt(const ObjCAutoreleasePoolStmt &S);
 
   void EmitCoroutineBody(const CoroutineBodyStmt &S);
+  void EmitCoreturnStmt(const CoreturnStmt &S);
   RValue EmitCoroutineIntrinsic(const CallExpr *E, unsigned int IID);
 
   void EnterCXXTryStmt(const CXXTryStmt &S, bool IsFnTryBlock = false);