[lld] Migrate away from PointerUnion::dyn_cast (NFC) (llvm#124504)

Note that PointerUnion::dyn_cast has been soft deprecated in
PointerUnion.h:

  // FIXME: Replace the uses of is(), get() and dyn_cast() with
  //        isa<T>, cast<T> and the llvm::dyn_cast<T>

This patch migrates uses of PointerUnion::dyn_cast to
dyn_cast_if_present (see the definition of PointerUnion::dyn_cast).
Note that we cannot use dyn_cast in any of the migrations in this
patch; placing

  assert(!X.isNull());

just before any of dyn_cast_if_present in this patch triggers some
failure in check-lld.

Author: kazutakahirata

lld/MachO/ObjC.cpp

@@ -543,7 +543,7 @@ ObjcCategoryMerger::tryGetSymbolAtIsecOffset(const ConcatInputSection *isec,
   if (!reloc)
     return nullptr;
 
-  Symbol *sym = reloc->referent.dyn_cast<Symbol *>();
+  Symbol *sym = dyn_cast_if_present<Symbol *>(reloc->referent);
 
   if (reloc->addend && sym) {
     assert(isa<Defined>(sym) && "Expected defined for non-zero addend");

lld/MachO/Writer.cpp

@@ -711,7 +711,7 @@ void Writer::scanRelocations() {
 
       // Canonicalize the referent so that later accesses in Writer won't
       // have to worry about it.
-      if (auto *referentIsec = r.referent.dyn_cast<InputSection *>())
+      if (auto *referentIsec = dyn_cast_if_present<InputSection *>(r.referent))
         r.referent = referentIsec->canonical();
 
       if (target->hasAttr(r.type, RelocAttrBits::SUBTRAHEND)) {
@@ -725,7 +725,7 @@ void Writer::scanRelocations() {
           it->referent = referentIsec->canonical();
         continue;
       }
-      if (auto *sym = r.referent.dyn_cast<Symbol *>()) {
+      if (auto *sym = dyn_cast_if_present<Symbol *>(r.referent)) {
         if (auto *undefined = dyn_cast<Undefined>(sym))
           treatUndefinedSymbol(*undefined, isec, r.offset);
         // treatUndefinedSymbol() can replace sym with a DylibSymbol; re-check.