IRGen: use a more portable comment leader

MachO uses ;, other targets use #.  This would result in a rather
cryptic error message when building libdispatch for Linux AArch64:

    <inline asm>:1:27: error: unexpected token in argument list
            mov     fp, fp          ; marker for objc_retainAutoreleaseReturnValue
                                                 ^
    LLVM ERROR: Error parsing inline asm

Author: compnerd

lib/IRGen/SwiftTargetInfo.cpp

@@ -50,7 +50,7 @@ static void configureARM64(IRGenModule &IGM, const llvm::Triple &triple,
 
   // arm64 requires marker assembly for objc_retainAutoreleasedReturnValue.
   target.ObjCRetainAutoreleasedReturnValueMarker =
-    "mov\tfp, fp\t\t; marker for objc_retainAutoreleaseReturnValue";
+    "mov\tfp, fp\t\t# marker for objc_retainAutoreleaseReturnValue";
 
   // arm64 requires ISA-masking.
   target.ObjCUseISAMask = true;