[LLD][COFF] Parallel sort publics

[aganea]

Summary

On large PDBs with many symbols, this can save up to 1 sec. On smaller PDBs, this saves time in all cases.

	Trunk	This patch
Large EXE (PDB is ~2 GB)	3330 ms	2022 ms
Large EXE (PDB is ~2 GB)	2680 ms	1608 ms
Large DLL (PDB is ~1 GB)	1455 ms	938 ms
Large DLL (PDB is ~800 MB)	1215 ms	800 ms
Small DLL (PDB is ~200 MB)	224 ms	146 ms

[aganea]

COFF/PDB.cpp

217

NOTE: Including Parallel.h creates these conflicts (which I can commit separately).

[rnk]

lgtm

COFF/PDB.cpp

 #include "llvm/DebugInfo/PDB/PDB.h"
 #include "llvm/Object/COFF.h"
 #include "llvm/Object/CVDebugRecord.h"
 #include "llvm/Support/BinaryByteStream.h"
 #include "llvm/Support/Endian.h"
 #include "llvm/Support/Errc.h"
 #include "llvm/Support/FormatVariadic.h"
 #include "llvm/Support/JamCRC.h"
+#include "llvm/Support/Parallel.h"
 #include "llvm/Support/Path.h"
 #include "llvm/Support/ScopedPrinter.h"
 #include <memory>
 
 using namespace lld;
 using namespace lld::coff;
 using namespace llvm;
 using namespace llvm::codeview;
private:
   /// A list of other PDBs which are loaded during the linking process and which
   /// we need to keep around since the linking operation may reference pointers
   /// inside of these PDBs.
   llvm::SmallVector<std::unique_ptr<pdb::NativeSession>, 2> LoadedPDBs;
 
   std::vector<pdb::SecMapEntry> SectionMap;
 
   /// Type index mappings of type server PDBs that we've loaded so far.
   std::map<codeview::GUID, CVIndexMap> TypeServerIndexMappings;
 
   /// Type index mappings of precompiled objects type map that we've loaded so
   /// far.
   std::map<uint32_t, CVIndexMap> PrecompTypeIndexMappings;
 
   /// List of TypeServer PDBs which cannot be loaded.
   /// Cached to prevent repeated load attempts.
   std::map<codeview::GUID, std::string> MissingTypeServerPDBs;
Symtab->forEachSymbol([&Publics](Symbol *S) {
     // Only emit defined, live symbols that have a chunk.
     auto *Def = dyn_cast<Defined>(S);
     if (Def && Def->isLive() && Def->getChunk())
       Publics.push_back(createPublic(Def));
   });
 
   if (!Publics.empty()) {
     // Sort the public symbols and add them to the stream.
-    std::sort(Publics.begin(), Publics.end(),
-              [](const PublicSym32 &L, const PublicSym32 &R) {
-                return L.Name < R.Name;
-              });
+    sort(parallel::par, Publics.begin(), Publics.end(),
     for (const PublicSym32 &Pub : Publics)
       GsiBuilder.addPublicSymbol(Pub);
   }
 }
 
 void PDBLinker::addNatvisFiles() {
   for (StringRef File : Config->NatvisFiles) {
     ErrorOr<std::unique_ptr<MemoryBuffer>> DataOrErr =