Add a clang 18 bot and move non-pinned bots to 18

[danakj]

No description provided.

[danakj]

This will require a roll of fmtlib after fmtlib/fmt#3607 is addressed.

[danakj]

LLVM reverted the change that enabled -Wdeprecated-literal-operator by default for now.

commit 0d9919d362a7a70b2a7970861d897ecc47ec9e4d
Author: Reid Kleckner <rnk@google.com>
Date:   Tue Aug 22 18:10:41 2023 -0700

    Revert "[Clang] CWG1473: do not err on the lack of space after operator"""

    This reverts commit f2583f3acf596cc545c8c0e3cb28e712f4ebf21b.

    There is a large body of non-conforming C-like code using format strings
    like this:

      #define PRIuS "zu"
      void h(size_t foo, size_t bar) {
        printf("foo is %"PRIuS", bar is %"PRIuS, foo, bar);
      }

    Rejecting this code would be very disruptive. We could decide to do
    that, but it's sufficiently disruptive that I think it requires
    gathering more community consensus with an RFC, and Aaron indicated [1]
    it's OK to revert for now so continuous testing systems can see past
    this issue while we decide what to do.

    [1] https://reviews.llvm.org/D153156#4607717