[coroutines] Refactor SuspendExpr to create just one OpaqueValue (almost NFC)

[coroutines] Refactor SuspendExpr to create just one OpaqueValue (almost NFC)

Summary:
Create only one OpaqueValue for await_ready/await_suspend/await_resume.
Store OpaqueValue used in the CoroutineSuspendExpr node, so that CodeGen does not have to hunt looking for it.

Reviewers: rsmith, EricWF, aaron.ballman

Reviewed By: EricWF

Subscribers: mehdi_amini, cfe-commits

Differential Revision: https://reviews.llvm.org/D30775

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@297541 91177308-0d34-0410-b5e6-96231b3b80d8