[llvm] [AccelTable] Pre-compute entry pool offsets for DW_IDX_parent. NFC (PR #188915)

[Jonas Devlieghere via llvm-commits]

JDevlieghere wrote:

Thank you @MaskRay! I reran my "benchmark" and can confirm that this addresses the issue.

Baseline:
```
$ time /Users/jonas/llvm/build-release/bin/dsymutil /Users/jonas/llvm/build-ra/bin/clang -o /tmp/clang.dSYM
________________________________________________________
Executed in   67.80 mins    fish           external
   usr time   68.51 mins    2.10 millis   68.51 mins
   sys time    0.12 mins    6.83 millis    0.12 mins
```

This PR applied:
```
$ time /Users/jonas/llvm/build-release/bin/dsymutil /Users/jonas/llvm/build-ra/bin/clang -o /tmp/clang.dSYM
________________________________________________________
Executed in  157.43 secs    fish           external
   usr time  201.57 secs    2.00 millis  201.57 secs
   sys time    4.73 secs    6.55 millis    4.72 secs
```

https://github.com/llvm/llvm-project/pull/188915