[ELF] Demote symbols in /DISCARD/ discarded sections to Undefined (#69295)

When an input section is matched by /DISCARD/ in a linker script, GNU ld
reports errors for relocations referencing symbols defined in the section:

    `.aaa' referenced in section `.bbb' of a.o: defined in discarded section `.aaa' of a.o

Implement the error by demoting eligible symbols to `Undefined` and changing
STB_WEAK to STB_GLOBAL. As a side benefit, in relocatable links, relocations
referencing symbols defined relative to /DISCARD/ discarded sections no longer
set symbol/type to zeros.

It's arguable whether a weak reference to a discarded symbol should lead to
errors. GNU ld reports an error and our demoting approach reports an error as
well.

Close #58891

Co-authored-by: Bevin Hansson <bevin.hansson@ericsson.com>

lld/ELF/LinkerScript.cpp

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