[lldb] Migrate away from PointerUnion::{is,get} (NFC) (#122420)

Note that PointerUnion::{is,get} have 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>

GitOrigin-RevId: 008a39c0e3f934c7eb0dd04aa5759a0feac65967
1 file changed