[Preprocessor] Allow libc++ to detect when aligned allocation is unavailable.

Libc++ needs to know when aligned allocation is supported by clang, but is
otherwise unavailable at link time. Otherwise, libc++ will incorrectly end up
generating calls to `__builtin_operator_new`/`__builtin_operator_delete` with
alignment arguments.

This patch implements the following changes:

* The `__cpp_aligned_new` feature test macro to no longer be defined when
  aligned allocation is otherwise enabled but unavailable.

* The Darwin driver no longer passes `-faligned-alloc-unavailable` when the
  user manually specifies `-faligned-allocation` or `-fno-aligned-allocation`.

* Instead of a warning Clang now generates a hard error when an aligned
  allocation or deallocation function is referenced but unavailable.

Patch by Eric Fiselier.

Reviewers: rsmith, vsapsai, erik.pilkington, ahatanak, dexonsmith

Reviewed By: rsmith

Subscribers: Quuxplusone, cfe-commits

Differential Revision: https://reviews.llvm.org/D45015

llvm-svn: 338934