The pipe signal handler must be installed before any other handlers are
registered. This is because the Unix RegisterHandlers function does not
perform a sigaction() for SIGPIPE unless a one-shot handler is present,
to allow long-lived processes (like lldb) to fully opt-out of llvm's
SIGPIPE handling and ignore the signal safely.
Fixes a bug introduced in D70277.
Tested by running Nick's test case:
% xcrun ./bin/clang -E -fno-integrated-cc1 x.c | tee foo.txt | head
I verified that child cc1 process exits with IO_ERR, and that the parent
recognizes the error code, exiting cleanly.