Replace a destructor of EHCleanupScope with a Destroy() method to reflect the current usage.

Summary:
The current code uses memset to re-initialize EHCleanupScope objects
with breaks the assumptions of the upcoming asan's intra-object-overflow checker.
If there is no DTOR, the new checker will refuse to work.

Test Plan: bootstrap with asan

Reviewers: rnk

Reviewed By: rnk

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D5656