[LLD][ELF] Rename variables and add comments to getISThunkSec [NFC]

[peter.smith]

Summary

Some refactoring split out from D34691 that isn't strictly relating to it. It covers the renaming of some variables as Commands were moved to OutputSections the existing ones still imply commands. I've also tidied up the comments.

[smeenai]

ELF/Relocations.cpp

1028

As far as I can see OS isn't used in this function anywhere. Can that parameter be dropped entirely? (I'm fine with that being done in a follow-up if it's possible.)

[peter.smith]

I've removed the unused parameter and have searched for other locations where Cmd is used instead of OS.

[smeenai]

Looks good to me still.

[ruiu]

LGTM

ELF/Relocations.cpp

ThunkSection *ThunkCreator::getOSThunkSec(OutputSection *Cmd,
                                           std::vector<InputSection *> *ISR) {
   if (CurTS == nullptr) {
     uint32_t Off = findEndOfFirstNonExec(*Cmd);
     CurTS = addThunkSection(Cmd, ISR, Off);
   }
   return CurTS;
 }
 
+// Add a Thunk that needs to be placed in a ThunkSection that immediately
+// precedes its Target.
 ThunkSection *ThunkCreator::getISThunkSec(InputSection *IS, OutputSection *OS) {
   ThunkSection *TS = ThunkedSections.lookup(IS);
   if (TS)
     return TS;
 
-  // Find InputSectionRange within TOS that IS is in
-  OutputSection *C = IS->getParent();
+  // Find InputSectionRange within Target Output Section (TOS) that the
+  // InputSection (IS) that we need to precede is in.
+  OutputSection *TOS = IS->getParent();
   std::vector<InputSection *> *Range = nullptr;
-  for (BaseCommand *BC : C->Commands)
+  for (BaseCommand *BC : TOS->Commands)
     if (auto *ISD = dyn_cast<InputSectionDescription>(BC)) {
       InputSection *first = ISD->Sections.front();
       InputSection *last = ISD->Sections.back();
       if (IS->OutSecOff >= first->OutSecOff &&
           IS->OutSecOff <= last->OutSecOff) {
         Range = &ISD->Sections;
         break;
       }
     }
-  TS = addThunkSection(C, Range, IS->OutSecOff);
+  TS = addThunkSection(TOS, Range, IS->OutSecOff);
   ThunkedSections[IS] = TS;
   return TS;
 }
 
 ThunkSection *ThunkCreator::addThunkSection(OutputSection *Cmd,
                                             std::vector<InputSection *> *ISR,
                                             uint64_t Off) {
   auto *TS = make<ThunkSection>(Cmd, Off);