[Driver] check for exit code from SIGPIPE
[Driver] check for exit code from SIGPIPE

Summary:
D53000 adds a special exit code for SIGPIPE (writing to a closed
reader), and rather than print a fatal warning, skips printing the
error.  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: cfe-commits, thakis, srhines

Differential Revision: https://reviews.llvm.org/D53001

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@344375 91177308-0d34-0410-b5e6-96231b3b80d8

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