CFG: Add CFGElement for automatic variables that leave the scope

Summary:
This mimics the implementation for the implicit destructors. The
generation of this scope leaving elements is hidden behind
a flag to the CFGBuilder, thus it should not affect existing code.

Currently, I'm missing a test (it's implicitly tested by the clang-tidy
lifetime checker that I'm proposing).
I though about a test using debug.DumpCFG, but then I would
have to add an option to StaticAnalyzer/Core/AnalyzerOptions
to enable the scope leaving CFGElement,
which would only be useful to that particular test.

Any other ideas how I could make a test for this feature?

Reviewers: krememek, jordan_rose

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D15031

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@307759 91177308-0d34-0410-b5e6-96231b3b80d8