[IRGen] Fetch the correct IGM for synthesized files

rdar://128870792

Synthesized files are treated as not having a parent source file, but that can cause issues for macro code. The declarations will have the source file of their use site declared as parent source file, which means they should be emitted into the same IGM. If we don't use the correct IGM, we can get inconsistencies when referencing the generated code, which will cause linking issues.

Author: drexin

lib/IRGen/GenDecl.cpp

@@ -2563,6 +2563,8 @@ void swift::irgen::disableAddressSanitizer(IRGenModule &IGM, llvm::GlobalVariabl
 
 /// Emit a global declaration.
 void IRGenModule::emitGlobalDecl(Decl *D) {
+  assert(this == irgen.getGenModule(D) &&
+         "Current module does not match module of decl");
   if (!D->isAvailableDuringLowering())
     return;
 

lib/IRGen/IRGen.cpp

@@ -1440,7 +1440,7 @@ static void performParallelIRGeneration(IRGenDescriptor desc) {
       }
 
       if (auto *synthSFU = File->getSynthesizedFile()) {
-        CurrentIGMPtr IGM = irgen.getGenModule(synthSFU);
+        CurrentIGMPtr IGM = irgen.getGenModule(&synthSFU->getFileUnit());
         IGM->emitSynthesizedFileUnit(*synthSFU);
       }
     } else {