[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

llvm-svn: 297541