[lld][ELF] Add --debug-names to create merged .debug_names. (#86508)

`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.

Co-authored-by: Fangrui Song <i@maskray.me>

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Co-authored-by: Fangrui Song <i@maskray.me>