[lld] Migrate away from PointerUnion::dyn_cast (NFC) (#125458)

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>

Literal migration would result in dyn_cast_if_present (see the
definition of PointerUnion::dyn_cast), but this patch uses dyn_cast
because we expect referent to be nonnull.

Author: kazutakahirata

lld/MachO/Relocations.cpp

@@ -32,7 +32,7 @@ InputSection *Reloc::getReferentInputSection() const {
 }
 
 StringRef Reloc::getReferentString() const {
-  if (auto *isec = referent.dyn_cast<InputSection *>()) {
+  if (auto *isec = dyn_cast<InputSection *>(referent)) {
     const auto *cisec = dyn_cast<CStringInputSection>(isec);
     assert(cisec && "referent must be a CStringInputSection");
     return cisec->getStringRefAtOffset(addend);