[Clang][ThinLTO] Wire up AllocToken pass to run in ThinLTO backend

[melver]

The AllocToken pass was running in the Pre-Link pipeline, which caused a
conflict with PGHO in ThinLTO builds because AllocToken rewrites
allocations (e.g. _Znam -> __alloc_token__Znam) before PGHO can see
it in the backend, preventing optimization.

Defer the execution of AllocTokenPass to the LTO Backend (Post-Link)
when ThinLTO is enabled by:

1. Adding a PassBuilderCallback hook to lto::Config and invoking it
   in LTOBackend.cpp.

2. Using this hook in BackendUtil.cpp to register AllocTokenPass
   into the LTO backend pipeline via registerOptimizerLastEPCallback.

3. Skipping AllocTokenPass in the Pre-Link phase if
   PrepareForThinLTO is set.

This ensures PGHO optimization (in InstCombine) runs first in the
backend, followed by AllocToken instrumentation on hot/cold new.

[github-actions]

🐧 Linux x64 Test Results

• 192893 tests passed
• 6194 tests skipped

[melver]

Superseded by #169360