[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)) {
    |                 ^~~~~~~~~~~

Author: mikaelholmen

lld/MachO/Driver.cpp

@@ -528,7 +528,7 @@ static void compileBitcodeFiles() {
   // FIXME: Remove this once LTO.cpp honors config->exportDynamic.
   if (config->exportDynamic)
     for (InputFile *file : inputFiles)
-      if (auto *bitcodeFile = dyn_cast<BitcodeFile>(file)) {
+      if (isa<BitcodeFile>(file)) {
         warn("the effect of -export_dynamic on LTO is not yet implemented");
         break;
       }