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

rdar://131352085

This change caused the problems it was supposed to fix for Macros to appear with Differentiation.

This reverts commit d33f2c4.

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 {