[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

llvm-svn: 344375