[lld] Replace SmallSet with SmallPtrSet (NFC) (#154263)

This patch replaces SmallSet<T *, N> with SmallPtrSet<T *, N>.  Note
that SmallSet.h "redirects" SmallSet to SmallPtrSet for pointer
element types:

  template <typename PointeeType, unsigned N>
class SmallSet<PointeeType*, N> : public SmallPtrSet<PointeeType*, N>
{};

We only have 30 instances that rely on this "redirection".  Since the
redirection doesn't improve readability, this patch replaces SmallSet
with SmallPtrSet for pointer element types.

I'm planning to remove the redirection eventually.

Author: kazutakahirata

lld/ELF/Relocations.cpp

@@ -273,12 +273,12 @@ template <class ELFT> static bool isReadOnly(SharedSymbol &ss) {
 // them are copied by a copy relocation, all of them need to be copied.
 // Otherwise, they would refer to different places at runtime.
 template <class ELFT>
-static SmallSet<SharedSymbol *, 4> getSymbolsAt(Ctx &ctx, SharedSymbol &ss) {
+static SmallPtrSet<SharedSymbol *, 4> getSymbolsAt(Ctx &ctx, SharedSymbol &ss) {
   using Elf_Sym = typename ELFT::Sym;
 
   const auto &file = cast<SharedFile>(*ss.file);
 
-  SmallSet<SharedSymbol *, 4> ret;
+  SmallPtrSet<SharedSymbol *, 4> ret;
   for (const Elf_Sym &s : file.template getGlobalELFSyms<ELFT>()) {
     if (s.st_shndx == SHN_UNDEF || s.st_shndx == SHN_ABS ||
         s.getType() == STT_TLS || s.st_value != ss.value)