[coroutines] Fix spills of static array allocas
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.
Committed by Gor Nishanov on behalf of ben-clayton
Reviewers: GorNishanov, modocache
Reviewed By: GorNishanov
Subscribers: Orlando, capn, EricWF, llvm-commits
Differential Revision: https://reviews.llvm.org/D61372