Recommit "[Support] change StringMap hash function from djbHash to xxHash"

This reverts commit 37eb9d13.

Test failures have been fixed:

- ubsan failure fixed by 72eac42f
- warn-unsafe-buffer-usage-fixits-local-var-span.cpp fixed by
  03cc52df (wasn't related)
- test-output-format.ll failure was spurious, build failed at
  https://lab.llvm.org/buildbot/#/builders/54/builds/3545 (b4431b2d)
  but passed at
  https://lab.llvm.org/buildbot/#/builders/54/builds/3546 (5ae99be0)
  which is before my revert
  https://github.com/llvm/llvm-project/compare/b4431b2d945b6fc19b1a55ac6ce969a8e06e1e93...5ae99be0377248c74346096dc475af254a3fc799

Original commit message:

    Depends on https://reviews.llvm.org/D142861.

    Alternative to https://reviews.llvm.org/D137601.

    xxHash is much faster than djbHash. This makes a simple Rust test case with a large constant string 10% faster to compile.

    Previous attempts at changing this hash function (e.g. https://reviews.llvm.org/D97396) had to be reverted due to breaking tests that depended on iteration order.
    No additional tests fail with this patch compared to `main` when running `check-all` with `-DLLVM_ENABLE_PROJECTS="all"` (on a Linux host), so I hope I found everything that needs to be changed.

    Differential Revision: https://reviews.llvm.org/D142862