[lld][WebAssemlby] Fix for string merging of -dwarf-5 sections

[sbc100]

Summary

We were mistakenly treating .debug_str_offsets as a string mergable
section when it is not (it contains integers not strings). This is an
indication that we really should find a way to store flags for custom
sections.

[sbc100]

• add bug links

[sbc100]

• add test

[dschuff]

lld/wasm/InputFiles.cpp

375

I guess there is no .debug_line_str.dwo because the .debug_line section goes into the main executable, instead of a dwo file?

[sbc100]

lld/wasm/InputFiles.cpp

375

Indeed. At least there is no such section marked in initWasmMCObjectFileInfo which is the canonical list of string merge-able sections (which is why I mention it here).

lld/wasm/InputFiles.cpp

 
 static bool shouldMerge(const WasmSection &sec) {
   if (config->optimize == 0)
     return false;
   // Sadly we don't have section attributes yet for custom sections, so we
   // currently go by the name alone.
   // TODO(sbc): Add ability for wasm sections to carry flags so we don't
   // need to use names here.
-  return sec.Name.startswith(".debug_str") ||
-         sec.Name.startswith(".debug_line_str");
+  // For now, keep in sync with uses of wasm::WASM_SEG_FLAG_STRINGS in
+  // MCObjectFileInfo::initWasmMCObjectFileInfo which creates these custom
+  // sections.
+  return sec.Name == ".debug_str" || sec.Name == ".debug_str.dwo" ||
+         sec.Name == ".debug_line_str";
 }
 
 static bool shouldMerge(const WasmSegment &seg) {
   // As of now we only support merging strings, and only with single byte
   // alignment (2^0).
   if (!(seg.Data.LinkingFlags & WASM_SEG_FLAG_STRINGS) ||
       (seg.Data.Alignment != 0))
     return false;