[analyzer] Warn about -analyzer-configs being meant for development purposes only

This is more of a temporary fix, long term, we should convert AnalyzerOptions.def
into the universally beloved (*coughs*) TableGen format, where they can more
easily be separated into developer-only, alpha, and user-facing configs.

Differential Revision: https://reviews.llvm.org/D66261

llvm-svn: 368980