[analyzer] Make CallEvent a value object.

We will need to be able to easily reconstruct a CallEvent from an ExplodedNode
for diagnostic purposes, and that's exactly what factory functions are for.
CallEvent objects are small enough (four pointers and a SourceLocation) that
returning them through the stack is fairly cheap. Clients who just need to use
existing CallEvents can continue to do so using const references.

This uses the same sort of "kind-field-dispatch" as SVal, though most of the
nastiness is contained in the DISPATCH and DISPATCH_ARG macros at the end of
the file. (We can't use a template for this because member-pointers to base
class methods don't call derived-class methods even when casting to the
derived class. We can't use variadic macros because they're a C99 feature.)

llvm-svn: 160459