Fix sed -e s@FOO@%/S@ and similar when there's @'s in the working directory
Jenkins sometimes starts a new working directory by appending @2 (or
incrementing the number if the @n suffix is already there). This causes
several clang tests to fail as:
gets expanded to the invalid:
where the part marked with ~'s is interpreted as the flags. These are
invalid and the test fails.
Previous fixes simply exchanged the @ character for another like | but
that's just moving the problem. Address it by adding an expansion that
escapes the @ character we're using as a delimiter as well as other magic
characters in the replacement of sed's s@@@.
There's still room for expansions to cause trouble though. One I ran into
while testing this was that having a directory called foo@bar causes lots
of CHECK-NOT: foo directives to match. There's also things like
directories containing \1