[analyzer] Expose return statement from CallExit program point

If the return statement is stored, we might as well allow querying
against it.
Also fix the bug where the return statement is not stored
if there is no return value.
This change un-merges two ExplodedNodes during call exit when the state
is otherwise identical - the CallExitBegin node itself and the "Bind
Return Value"-tagged node.
And expose the return statement through
getStatement helper function.

Differential Revision: https://reviews.llvm.org/D42130

llvm-svn: 324052