[clang][NFC] Small mangler cleanups

In working on a module mangling problem I noticed a few cleanups to the mangler.

1) Use 'if (auto x = ...' idiom in a couple of places.

2) I noticed both 'isFileContext' and 'isNamespace || isTranslationUnit'
   synonyms. Let's use the former.

3) The control flow in the seqId mangling was misordered. Let's channel Count
   von Count. Also fix the inconsistent bracing.

Differential Revision: https://reviews.llvm.org/D117799