[analyzer][NFC] Fix inconsistencies in AnalyzerOptions

I'm in the process of refactoring AnalyzerOptions. The main motivation behind
here is to emit warnings if an invalid -analyzer-config option is given from the
command line, and be able to list them all.

This first NFC patch contains small modifications to make AnalyzerOptions.cpp a
little more consistent.

Differential Revision: https://reviews.llvm.org/D53274

llvm-svn: 344870