[analyzer] Replace bug category magic strings with shared constants, take 2.

Re-commit r191910 (reverted in r191936) with layering violation fixed, by
moving the bug categories to StaticAnalyzerCore instead of ...Checkers.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@191937 91177308-0d34-0410-b5e6-96231b3b80d8