[clang][analyzer][NFC] Refactor code of StdLibraryFunctionsChecker.

The code was difficult to maintain (big internal class definitions
with long inline functions, other functions of the same class at
different location far away, irregular ordering of classes and
function definitions). It is now improved to some extent.
New functions are added to RangeConstraint to remove code repetition,
these are useful for planned new features too.
Comments are improved.

Reviewed By: Szelethus

Differential Revision: https://reviews.llvm.org/D143751