[GSYM] Add Split Dwarf Support to DwarfTransformer

[ayermolo]

Summary

The convert only worked on CUs in main binary.
If it's a skeleton CU it will now use the DWO CU
when invoking handleDie.

Test Plan:
llvm-lit

[clayborg]

I can help come up with a test for this before we commit it.

llvm/lib/DebugInfo/GSYM/DwarfTransformer.cpp

440

Fix indent per pre-merge above

[clayborg]

So we need a test for this. All we need to do is to create a .o file with .dwo sections inlined into it and then test that we can successfully run llvm-gsymutil on it. So if we can get a .o file with .dwo inlined, then we run obj2yaml and verify that yaml2obj successfully reconstructs the .o file intact (hoping that obj2yaml handles all DWARF sections including the ones for .dwo), then we can make a test case very similar to this one:

llvm/test/tools/llvm-gsymutil/X86/elf-dwarf.yaml

This test has the yaml inlined into it and then runs FileCheck on the resulting llvm-gsymutil invocations.

[ayermolo]

added test

[clayborg]

Thanks for adding the test!

llvm/lib/DebugInfo/GSYM/DwarfTransformer.cpp

default:
     break;
   }
   for (DWARFDie ChildDie : Die.children())
     handleDie(OS, CUI, ChildDie);
 }
 
 Error DwarfTransformer::convert(uint32_t NumThreads) {
   size_t NumBefore = Gsym.getNumFunctionInfos();
+  auto getDie = [&](DWARFUnit &DwarfUnit) -> DWARFDie {
+    DWARFDie ReturnDie = DwarfUnit.getUnitDIE(false);
+    if (llvm::Optional<uint64_t> DWOId = DwarfUnit.getDWOId()) {
+      DWARFUnit *DWOCU = DwarfUnit.getNonSkeletonUnitDIE(false).getDwarfUnit();
+      if (!DWOCU->isDWOUnit()) {
+        std::string DWOName = dwarf::toString(
+            DwarfUnit.getUnitDIE().find(
+                {dwarf::DW_AT_dwo_name, dwarf::DW_AT_GNU_dwo_name}),
+            "");
+        Log << "warning: Unable to retrieve DWO .debug_info section for "
+              << DWOName << "\n";
+      } else {
+        ReturnDie = DWOCU->getUnitDIE(false);
+      }
+    }
+    return ReturnDie;
+  };
   if (NumThreads == 1) {
     // Parse all DWARF data from this thread, use the same string/file table
     // for everything
     for (const auto &CU : DICtx.compile_units()) {
-      DWARFDie Die = CU->getUnitDIE(false);
+      DWARFDie Die = getDie(*CU.get());
       CUInfo CUI(DICtx, dyn_cast<DWARFCompileUnit>(CU.get()));
       handleDie(Log, CUI, Die);
     }
   } else {
     // LLVM Dwarf parser is not thread-safe and we need to parse all DWARF up
     // front before we start accessing any DIEs since there might be
     // cross compile unit references in the DWARF. If we don't do this we can
     // end up crashing.
 
     // We need to call getAbbreviations sequentially first so that getUnitDIE()
     // only works with its local data.
     for (const auto &CU : DICtx.compile_units())
       CU->getAbbreviations();
 
     // Now parse all DIEs in case we have cross compile unit references in a
     // thread pool.
     ThreadPool pool(hardware_concurrency(NumThreads));
     for (const auto &CU : DICtx.compile_units())
       pool.async([&CU]() { CU->getUnitDIE(false /*CUDieOnly*/); });
     pool.wait();
 
     // Now convert all DWARF to GSYM in a thread pool.
     std::mutex LogMutex;
     for (const auto &CU : DICtx.compile_units()) {
-      DWARFDie Die = CU->getUnitDIE(false /*CUDieOnly*/);
+      DWARFDie Die = getDie(*CU.get());
       if (Die) {
         CUInfo CUI(DICtx, dyn_cast<DWARFCompileUnit>(CU.get()));
         pool.async([this, CUI, &LogMutex, Die]() mutable {
           std::string ThreadLogStorage;
           raw_string_ostream ThreadOS(ThreadLogStorage);
           handleDie(ThreadOS, CUI, Die);
           ThreadOS.flush();
           if (!ThreadLogStorage.empty()) {