[coroutines] Fix spills of static array allocas

Summary:
CoroFrame was not considering static array allocas, and was only ever reserving a single element in the coroutine frame.
This meant that stores to the non-zero'th element would corrupt later frame data.

Store static array allocas as field arrays in the coroutine frame.

Added test.

Committed by Gor Nishanov on behalf of ben-clayton
Reviewers: GorNishanov, modocache

Reviewed By: GorNishanov

Subscribers: Orlando, capn, EricWF, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D61372

llvm-svn: 360636