[analyzer][NFC] Keep track of whether enabling a checker was explictly...

[analyzer][NFC] Keep track of whether enabling a checker was explictly specified in command line arguments

I added a new enum to CheckerInfo, so we can easily track whether the check is
explicitly enabled, explicitly disabled, or isn't specified in this regard.
Checkers belonging in the latter category may be implicitly enabled through
dependencies in the followup patch. I also made sure that this is done within
CheckerRegisty's constructor, leading to very significant simplifications in
its query-like methods.

Differential Revision: https://reviews.llvm.org/D56989

llvm-svn: 352282