Fix strict-aliasing violation in typeinfo::hash_code()

Summary:
The current implementation of `hash_code()` for uniqued RTTI strings violates strict aliasing by dereferencing a type-punned pointer. Specifically it generates a `const char**` pointer from the address of the `__name` member before casting it to `const size_t*` and dereferencing it to get the hash. This is really just a complex and incorrect way of writing `reinterpret_cast<size_t>(__name)`.

This patch changes the conversion sequence so that it no longer contains UB.


Reviewers: howard.hinnant, mclow.lists

Subscribers: rjmccall, cfe-commits

Differential Revision: https://reviews.llvm.org/D24012

llvm-svn: 283408