[Format] Add format check for coroutine keywords with negative numbers

Summary:
As a followup to D69144, this diff fixes the coroutine keyword spacing
for co_yield / co_returning negative numbers.

Reviewers: modocache, sammccall, Quuxplusone

Reviewed By: modocache

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D69180

Patch by Jonathan Thomas (jonathoma)!