[lld/mac] Fix warning about unused variable [NFC]

Change "dyn_cast" to "isa" to get rid of the unused
variable "bitcodeFile".

gcc warned with

lld/MachO/Driver.cpp:531:17: warning: unused variable 'bitcodeFile' [-Wunused-variable]
531 |       if (auto *bitcodeFile = dyn_cast<BitcodeFile>(file)) {
    |                 ^~~~~~~~~~~