Add Clang 18 to the build matrix.

... and remove Clang 15 so that we continue to
test with only the three most recent releases.

Change-Id: Ib1eaa20b2c988bb53027dd3e01551d5d95058e24
Reviewed-on: https://code-review.googlesource.com/c/re2/+/62830
Reviewed-by: Perry Lorier <perryl@google.com>
Reviewed-by: Paul Wankadia <junyer@google.com>