[clang/asan] call __asan_poison_cxx_array_cookie after operator new[]

Summary:
PR19838
When operator new[] is called and an array cookie is created
we want asan to detect buffer overflow bugs that touch the cookie.
For that we need to
  a) poison the shadow for the array cookie (call __asan_poison_cxx_array_cookie).
  b) ignore the legal accesses to the cookie generated by clang (add 'nosanitize' metadata)

Reviewers: timurrrr, samsonov, rsmith

Reviewed By: rsmith

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D4774