[ELF] Merge copyLocalSymbols and demoteLocalSymbolsInDiscardedSections (#69425)

Follow-up to #69295: In `Writer<ELFT>::run`, the symbol passes are
flexible:
they can be placed almost everywhere before `scanRelocations`, with a
constraint
that the `computeIsPreemptible` pass must be invoked for linker-defined
non-local symbols.

Merge copyLocalSymbols and demoteLocalSymbolsInDiscardedSections to
simplify
code:

* Demoting local symbols can be made unconditional, not constrainted to
/DISCARD/ uses due to performance concerns
* `includeInSymtab` can be made faster
* Make symbol passes close to each other
* Decrease data cache misses due to saving an iteration over local
symbols

There is no speedup, likely due to the unconditional `dr->section`
access in `demoteAndCopyLocalSymbols`.

`gc-sections-tls.s` no longer reports an error because the TLS symbol is
converted to an Undefined.

lld/ELF/LinkerScript.cpp

@@ -613,7 +613,6 @@ void LinkerScript::processSectionCommands() {
         discard(*s);
       discardSynthetic(*osec);
       osec->commands.clear();
-      seenDiscard = true;
       return false;
     }