[analyzer] Don't allow hidden checkers to emit diagnostics

Hidden checkers (those marked with Hidden in Checkers.td) are meant for
development purposes only, and are only displayed under
-analyzer-checker-help-developer, so users shouldn't see reports from them.

I moved StdLibraryFunctionsArg checker to the unix package from apiModeling as
it violated this rule. I believe this change doesn't deserve a different
revision because it is in alpha, and the name is so bad anyways I don't
immediately care where it is, because we'll have to revisit it soon enough.

Differential Revision: https://reviews.llvm.org/D81750