Merge pull request #75247 from drexin/wip-131352085-6.0

[6.0] Revert "[IRGen] Fetch the correct IGM for synthesized files"

Author: drexin

lib/IRGen/IRGen.cpp

@@ -1431,7 +1431,7 @@ static void performParallelIRGeneration(IRGenDescriptor desc) {
       }
 
       if (auto *synthSFU = File->getSynthesizedFile()) {
-        CurrentIGMPtr IGM = irgen.getGenModule(&synthSFU->getFileUnit());
+        CurrentIGMPtr IGM = irgen.getGenModule(synthSFU);
         IGM->emitSynthesizedFileUnit(*synthSFU);
       }
     } else {