[ELF] - Optimization for populating stringpool when building .gdb_index.

[grimar]

Summary

It is possible to speedup population of string pool just
by reusing precalculated hash value.

Tested llc binary to link, 50 runs each:
Without patch, with --gdb-index: 7,826269134 seconds time elapsed ( +- 0,06% )
With patch, with --gdb-index: 7,729554651 seconds time elapsed ( +- 0,11% )
A / B == 1,0125, or at least 1% for free.

[grimar]

Irrelevant.

ELF/SyntheticSections.cpp

void GdbIndexSection::readDwarf(InputSection *Sec) {
   for (AddressEntry &Ent : readAddressArea(Dwarf, Sec, CuId))
     AddressArea.push_back(Ent);
 
   std::vector<std::pair<StringRef, uint8_t>> NamesAndTypes =
       readPubNamesAndTypes(Dwarf, Config->IsLE);
 
   for (std::pair<StringRef, uint8_t> &Pair : NamesAndTypes) {
     uint32_t Hash = hash(Pair.first);
-    size_t Offset = StringPool.add(Pair.first);
+    size_t Offset = StringPool.add({Pair.first, Hash});
 
     bool IsNew;
     GdbSymbol *Sym;
     std::tie(IsNew, Sym) = SymbolTable.add(Hash, Offset);
     if (IsNew) {
       Sym->CuVectorIndex = CuVectors.size();
       CuVectors.push_back({{CuId, Pair.second}});
       continue;