[libcxx] Rework sized delete.

Summary:
This patch does 2 main things:
1. Enable sized delete if the feature test macro `__cpp_sized_deallocation` is enabled.
2. Rework and cleanup all of the sized delete tests.

Test Plan:
The sized delete replacement tests are now split into 4 files:
1. sized_delete11.pass.cpp: Ensure overriding sized delete in C++11 has no effect.
2. sized_delete14.pass.cpp: Test overriding sized delete in C++14 and ensure it is called. This test fails on clang and GCC < 5.1. 
3. size_delete_calls_unsized_delete_.pass.cpp: Test that the default sized delete calls unsized delete.
4. sized_delete_fsizeddeallocation.pass.cpp: Test overriding sized delete when -fsized-deallocation is passed. This test should pass on clang and GCC >= 5.1

I have also removed a lot of cruft from the old tests. They no longer replace the new handler and tests that it is called for bad allocations.

Reviewers: mclow.lists

Reviewed By: mclow.lists

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D9831

llvm-svn: 237662