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:
  s@INPUT_DIR@%/S/Inputs@g
gets expanded to the invalid:
  s@INPUT_DIR@/path/to/workdir@2/Inputs@g
                               ~~~~~~~~~~
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`