[analysis] Introduce an AnyCall helper class, for abstraction over different callables

A lot of code, particularly in the analyzer, has to perform a lot of
duplication to handle functions/ObjCMessages/destructors/constructors in
a generic setting.
The analyzer already has a CallEvent helper class abstracting over such
calls, but it's not always suitable, since it's tightly coupled to other
analyzer classes (ExplodedNode, ProgramState, etc.) and it's not always
possible to construct.

This change introduces a very simple, very lightweight helper class to
do simple generic operations over callables.

In future, parts of CallEvent could be changed to use this class to
avoid some duplication.

Differential Revision: https://reviews.llvm.org/D57126

llvm-svn: 352148