[analyzer] NFC: Add a convenient CallDescriptionMap class.

It encapsulates the procedure of figuring out whether a call event
corresponds to a function that's modeled by a checker.

Checker developers no longer need to worry about performance of
lookups into their own custom maps.

Add unittests - which finally test CallDescription itself as well.

Differential Revision: https://reviews.llvm.org/D62441

llvm-svn: 364866