[lld] Migrate away from PointerUnion::dyn_cast (NFC) (llvm#123721)

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 cast
because we know expect isa<InputSection *>(reloc.referent) to be true.

Author: kazutakahirata

lld/MachO/Arch/ARM64.cpp

@@ -205,7 +205,7 @@ InputSection *ARM64::getThunkBranchTarget(InputSection *thunk) const {
   assert(isa<InputSection *>(reloc.referent) &&
          "ARM64 thunk reloc is expected to point to an InputSection");
 
-  return reloc.referent.dyn_cast<InputSection *>();
+  return cast<InputSection *>(reloc.referent);
 }
 
 uint32_t ARM64::getICFSafeThunkSize() const { return sizeof(icfSafeThunkCode); }