[lld] [lld][ELF] Implement merged .debug_names section. (PR #86508)

[Fangrui Song via llvm-commits]

MaskRay wrote:

I've added fine-grained `TimeTraceScope` to my branch. In a `-O3 -g -gpubnames` build of clang, I've measured
```
0.115245 Merge .debug_names parse
0.002858 Merge .debug_names hdr and abbrev table
0.71001 Merge .debug_names entry pool
0.875658 Merge .debug_names null
0.875657 Total Merge .debug_names null
```

I've made entry pool computation parallel. It's much faster now:
```
0.113866 Merge .debug_names parse
0.00294 Merge .debug_names hdr and abbrev table
0.211803 Merge .debug_names entry pool
0.39303 Merge .debug_names null
0.393029 Total Merge .debug_names null
```

https://github.com/llvm/llvm-project/pull/86508