[DWARFLinker] Only extract unit DIEs when cloning clang modules (llvm#69495)

Reduce memory usage by only extract unit DIEs when cloning clang
modules. We don't need the full debug info yet at this stage. This
reduces peak memory usage of dsymutil when linking the swift driver by
multiple gigabytes.

rdar://117156180

Author: JDevlieghere

llvm/lib/DWARFLinker/DWARFLinker.cpp

@@ -2682,12 +2682,12 @@ Error DWARFLinker::link() {
       continue;
     }
 
-    // In a first phase, just read in the debug info and load all clang modules.
+    // Clone all the clang modules with requires extracting the DIE units. We
+    // don't need the full debug info until the Analyze phase.
     OptContext.CompileUnits.reserve(
         OptContext.File.Dwarf->getNumCompileUnits());
-
     for (const auto &CU : OptContext.File.Dwarf->compile_units()) {
-      auto CUDie = CU->getUnitDIE(false);
+      auto CUDie = CU->getUnitDIE(/*ExtractUnitDIEOnly=*/true);
       if (Options.Verbose) {
         outs() << "Input compilation unit:";
         DIDumpOptions DumpOpts;
@@ -2728,9 +2728,9 @@ Error DWARFLinker::link() {
       return;
 
     for (const auto &CU : Context.File.Dwarf->compile_units()) {
-      // The !isClangModuleRef condition effectively skips over fully resolved
-      // skeleton units.
-      auto CUDie = CU->getUnitDIE();
+      // Previously we only extracted the unit DIEs. We need the full debug info
+      // now.
+      auto CUDie = CU->getUnitDIE(/*ExtractUnitDIEOnly=*/false);
       std::string PCMFile = getPCMFile(CUDie, Options.ObjectPrefixMap);
 
       if (!CUDie || LLVM_UNLIKELY(Options.Update) ||