[libc-commits] [libc] [libc][signal] clean up usage of sighandler_t (PR #125745)

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enh-google wrote:

shouldn't `__sighandler_t` come from the uapi header, and only if you're using the uapi headers? (so nothing except the uapi headers should ever refer to that type, and the uapi headers themselves should be fine because they'll take care of their own transitive includes.)

https://github.com/llvm/llvm-project/pull/125745