[Support] change StringMap hash function from djbHash to xxHash

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