[Format] Add format check for throwing negative numbers

Summary:
The code `throw -1;` is currently formatted by clang-format as
`throw - 1;`. This diff adds a fix for this edge case and a test to check
for this in the future.

For context, I am looking into a related bug in the clang-formatting of
coroutine keywords: `co_yield -1;` is also reformatted in this manner
as `co_yield - 1;`. A later diff will add these changes and tests for the
`co_yield` and `co_return` keywords.

Patch by Jonathan Thomas (jonathoma)!

Reviewers: modocache, sammccall, Quuxplusone

Reviewed By: sammccall

Subscribers: cfe-commits

Tags: #clang-format, #clang

Differential Revision: https://reviews.llvm.org/D69144

llvm-svn: 375258