[analyzer] Add support for namespaces to GenericTaintChecker

This patch introduces the namespaces for the configured functions and
also enables the use of the member functions.

I added an optional Scope field for every configured function. Functions
without Scope match for every function regardless of the namespace.
Functions with Scope will match if the full name of the function starts
with the Scope.
Multiple functions can exist with the same name.

Differential Revision: https://reviews.llvm.org/D70878