[NFC][Debugify] Format the CheckModuleDebugify output

This fixes the output of the check-debugify option.
Without the patch an example of running the option:

$ opt -check-debugify test.ll -S -o testDebugify.ll
CheckModuleDebugifySkipping module without debugify metadata

After the patch:

$ opt -check-debugify test.ll -S -o testDebugify.ll
CheckModuleDebugify: Skipping module without debugify metadata

Differential Revision: https://reviews.llvm.org/D80553

llvm/lib/Transforms/Utils/Debugify.cpp

@@ -293,7 +293,7 @@ bool checkDebugifyMetadata(Module &M,
   // Skip modules without debugify metadata.
   NamedMDNode *NMD = M.getNamedMetadata("llvm.debugify");
   if (!NMD) {
-    dbg() << Banner << "Skipping module without debugify metadata\n";
+    dbg() << Banner << ": Skipping module without debugify metadata\n";
     return false;
   }