[analyzer] Process state in checkEndFunction in RetainCountChecker

Modify the RetainCountChecker to perform state "adjustments" in
checkEndFunction, as performing work in PreStmt<ReturnStmt> does not
work with destructors.
The previous version made an implicit assumption that no code runs
after the return statement is executed.

rdar://43945028

Differential Revision: https://reviews.llvm.org/D52338

llvm-svn: 342770