Simplify and optimize .debug_names and removed dead TU code

[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.

Co-authored-by: Fangrui Song <[email protected]>

Author: MaskRay

lld/ELF/DWARF.cpp

@@ -40,7 +40,7 @@ template <class ELFT> LLDDwarfObj<ELFT>::LLDDwarfObj(ObjFile<ELFT> *obj) {
                 .Case(".debug_gnu_pubtypes", &gnuPubtypesSection)
                 .Case(".debug_line", &lineSection)
                 .Case(".debug_loclists", &loclistsSection)
-                .Case(".debug_names", &debugNamesSection)
+                .Case(".debug_names", &namesSection)
                 .Case(".debug_ranges", &rangesSection)
                 .Case(".debug_rnglists", &rnglistsSection)
                 .Case(".debug_str_offsets", &strOffsetsSection)