[coroutines] update coro_end builtin to match llvm

Summary: llvm.coro.end intrinsic now returns bool. Updating clang to match it.

Reviewers: GorNishanov, rsmith

Reviewed By: rsmith

Subscribers: mehdi_amini, cfe-commits, EricWF

Differential Revision: https://reviews.llvm.org/D30711

llvm-svn: 297224