In working on a module mangling problem I noticed a few cleanups to the mangler.
- Use 'if (auto x = ...' idiom in a couple of places.
- I noticed both 'isFileContext' and 'isNamespace || isTranslationUnit' synonyms. Let's use the former.
- The control flow in the seqId mangling was misordered. Let's channel Count von Count. Also fix the inconsistent bracing.