[ELF] Refactor how .gnu.hash and .hash are discarded

Switch to the D114180 approach which is simpler and allows gnuHashTab/hashTab to
switch to unique_ptr.

lld/ELF/LinkerScript.cpp

@@ -564,14 +564,6 @@ void LinkerScript::discard(InputSectionBase &s) {
   if (&s == in.shStrTab.get())
     error("discarding " + s.name + " section is not allowed");
 
-  // You can discard .hash and .gnu.hash sections by linker scripts.
-  // Since they are synthesized sections, we need to handle them differently
-  // than other regular sections.
-  if (&s == mainPart->gnuHashTab)
-    mainPart->gnuHashTab = nullptr;
-  else if (&s == mainPart->hashTab)
-    mainPart->hashTab = nullptr;
-
   s.markDead();
   s.parent = nullptr;
   for (InputSection *sec : s.dependentSections)

lld/ELF/SyntheticSections.cpp

@@ -1464,9 +1464,9 @@ DynamicSection<ELFT>::computeContents() {
   addInt(DT_STRSZ, part.dynStrTab->getSize());
   if (!config->zText)
     addInt(DT_TEXTREL, 0);
-  if (part.gnuHashTab)
+  if (part.gnuHashTab && part.gnuHashTab->getParent())
     addInSec(DT_GNU_HASH, *part.gnuHashTab);
-  if (part.hashTab)
+  if (part.hashTab && part.hashTab->getParent())
     addInSec(DT_HASH, *part.hashTab);
 
   if (isMain) {