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

[Fangrui Song via llvm-commits]

MaskRay wrote:

Suggested commit subject and message:

```
[ELF] Add --debug-names to create merged .debug_names

`clang -g -gpubnames` (with optional -gsplit-dwarf) creates the
`.debug_names` section ("per-CU" index). By default lld concatenates
input `.debug_names` sections into an output `.debug_names` section.
LLDB can consume the concatenated section but the lookup performance is
not good.

This patch adds --debug-names to create a per-module index by combining
the per-CU indexes into a single index that covers the entire load
module. The produced `.debug_names` is a replacement for `.gdb_index`.

Type units (-fdebug-types-section) are not handled yet.
```

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