[clang][analyzer] Merge apiModeling.StdCLibraryFunctions and...

[clang][analyzer] Merge apiModeling.StdCLibraryFunctions and StdCLibraryFunctionArgs checkers into one.

Main reason for this change is that these checkers were implemented in the same class
but had different dependency ordering. (NonNullParamChecker should run before StdCLibraryFunctionArgs
to get more special warning about null arguments, but the apiModeling.StdCLibraryFunctions was a modeling
checker that should run before other non-modeling checkers. The modeling checker changes state in a way
that makes it impossible to detect a null argument by NonNullParamChecker.)
To make it more simple, the modeling part is removed as separate checker and can be only used if
checker StdCLibraryFunctions is turned on, that produces the warnings too. Modeling the functions
without bug detection (for invalid argument) is not possible. The modeling of standard functions
does not happen by default from this change on.

Reviewed By: Szelethus

Differential Revision: https://reviews.llvm.org/D151225