[clang] [llvm] [Hashing] Use a non-deterministic seed if LLVM_ENABLE_ABI_BREAKING_CHECKS (PR #96282)

[Fangrui Song via cfe-commits]

MaskRay wrote:

> The premerge config seems broken because of this PR.
> 
> [lab.llvm.org/buildbot](https://lab.llvm.org/buildbot/#/builders/153/builds/1490)

(The two `lld/test/MachO` failures did not reliably reproduce. Fixed ~3 days ago.)

` BOLT::reader-stale-yaml-std.test` relied on particular hash values, which could not be made reliably passing. Disabled it in  58004e5bb74de607f2f81649fd5a8db3ee318528 and informed BOLT maintainers.

https://github.com/llvm/llvm-project/pull/96282