[clang][bytecode] Use tailcalls via `[[clang::musttail]]` (#173756)

This PR attempts to use a tailcall design for the bytecode interpreter
instead of the giant-switch design, similar to [what Python has been
doing](https://blog.reverberate.org/2025/02/10/tail-call-updates.html).

Using the two benchmarks from
https://developers.redhat.com/articles/2025/10/15/clang-bytecode-interpreter-update,
I get these numbers:
```
-- Sqlite
BASELINE:
Benchmark 1: bin/clang++ -c -std=c++26 sqlite.cpp -fconstexpr-steps=1000000000
  Time (mean ± σ):     18.197 s ±  0.370 s    [User: 17.926 s, System: 0.269 s]
  Range (min … max):   17.657 s … 19.215 s    30 runs

BEFORE:
Benchmark 1: bin/clang++ -c -std=c++26 sqlite.cpp -fconstexpr-steps=1000000000 -fexperimental-new-constant-interpreter
  Time (mean ± σ):      8.156 s ±  0.148 s    [User: 7.881 s, System: 0.273 s]
  Range (min … max):    7.909 s …  8.491 s    30 runs

AFTER:
Benchmark 1: bin/clang++ -c -std=c++26 sqlite.cpp -fconstexpr-steps=1000000000 -fexperimental-new-constant-interpreter
  Time (mean ± σ):      7.492 s ±  0.171 s    [User: 7.214 s, System: 0.277 s]
  Range (min … max):    7.213 s …  7.876 s    30 runs

-- Heap allocation:
BASELINE:
Benchmark 1: bin/clang++ -c -std=c++26 test.cpp -fconstexpr-steps=1000000000
  Time (mean ± σ):     28.650 s ±  0.475 s    [User: 28.627 s, System: 0.021 s]
  Range (min … max):   27.767 s … 29.554 s    30 runs


BEFORE:
Benchmark 1: bin/clang++ -c -std=c++26 test.cpp -fconstexpr-steps=1000000000 -fexperimental-new-constant-interpreter
  Time (mean ± σ):      4.543 s ±  0.194 s    [User: 4.527 s, System: 0.016 s]
  Range (min … max):    4.340 s …  5.143 s    30 runs

AFTER:
Benchmark 1: bin/clang++ -c -std=c++26 test.cpp -fconstexpr-steps=1000000000 -fexperimental-new-constant-interpreter
  Time (mean ± σ):      4.055 s ±  0.140 s    [User: 4.039 s, System: 0.015 s]
  Range (min … max):    3.827 s …  4.430 s    30 runs
```

However, the numbers on the compile-time tracker are slightly worse:
https://llvm-compile-time-tracker.com/compare.php?from=699354a4146e2ebc7e7c7aa84d14c396a4300508&to=0b9191c493c35d33721c9aad7ab341fe2a731f3f&stat=instructions:u