[lld] [ELF] Change .debug_names tombstone value to UINT32_MAX/UINT64_MAX (PR #74686)

[Fangrui Song via llvm-commits]

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@@ -898,10 +898,16 @@ void InputSection::relocateNonAlloc(uint8_t *buf, ArrayRef<RelTy> rels) {
   const TargetInfo &target = *elf::target;
   const auto emachine = config->emachine;
   const bool isDebug = isDebugSection(*this);
-  const bool isDebugLocOrRanges =
-      isDebug && (name == ".debug_loc" || name == ".debug_ranges");
   const bool isDebugLine = isDebug && name == ".debug_line";
-  std::optional<uint64_t> tombstone;
+  std::optional<uint64_t> tombstone, debugTombstone;
----------------
MaskRay wrote:

`type == target.symbolicRel` is a proxy for whether the relocation is 64-bit on ELF64 (`symbolicRel`).
Previously, the entries in debug sections that need tombstone values are all `symbolicRel` since they reference a text section (and for generalization, we don't use the small code model assumption).
`.debug_names` is a new special case where its `.long .Ltu_begin0 # Type unit 0` reference is 32-bit.

A natural extension of the existing code looks like this.

`-z dead-reloc-in-nonalloc=` was initially designed to support both 32-bit and 64-bit absolute relocations.

---

Actually, x86-64 is unique in discerning signed/unsigned 32-bit absolute relocations (R_X86_64_32/R_X86_64_32S).
We can special case it instead:
```
if (emachine == EM_X86_64 && type == R_X86_64_32)
  value = static_cast<uint32_t>(value);
```

(AArch64/PPC64/RISC-V/etc don't need this.)

I have made the simplication.


https://github.com/llvm/llvm-project/pull/74686