[Support] exit with custom return code for SIGPIPE

Summary:
We tell the user to file a bug report on LLVM right now, and
SIGPIPE isn't LLVM's fault so our error message is wrong.

Allows frontends to detect SIGPIPE from writing to closed readers.
This can be seen commonly from piping into head, tee, or split.

Fixes PR25349, rdar://problem/14285346, b/77310947

Reviewers: jfb

Reviewed By: jfb

Subscribers: majnemer, kristina, llvm-commits, thakis, srhines

Differential Revision: https://reviews.llvm.org/D53000

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@344372 91177308-0d34-0410-b5e6-96231b3b80d8