[libcxxabi] Delay adjustment of pointer to prevent referencing invalid memory.

Summary:
This patch delays the dereference adjustment until we are sure the thrown type is a pointer type. It is possible the thrown type is not a pointer and is smaller than `sizeof(void*)`. If the thrown type is is smaller than `sizeof(void*)` the deference adjustment will result in a heap buffer overflow.

I audited all the call sites of `can_catch(...)` and there are no places where `adjustedPtr` is used if `can_catch(...)` returns false. For this reason the patch should not introduce any functionality change.

This patch fixes the following tests when using ASAN:
* unwind_01.cpp
* unwind_02.cpp
* unwind_04.cpp



Reviewers: danalbert, jroelofs, mclow.lists

Reviewed By: mclow.lists

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D6353