[libc++] Remove various C++03 feature test macros

Summary:
Libc++ still uses per-feature configuration macros when configuring for C++11. However libc++ requires a feature-complete C++11 compiler so there is no reason to check individual features. This patch starts the process of removing the feature specific macros and replacing their usage with `_LIBCPP_CXX03_LANG`.

This patch removes the __config macros:

* _LIBCPP_HAS_NO_TRAILING_RETURN
* _LIBCPP_HAS_NO_TEMPLATE_ALIASES
* _LIBCPP_HAS_NO_ADVANCED_SFINAE
* _LIBCPP_HAS_NO_DEFAULT_FUNCTION_TEMPLATE_ARGS
* _LIBCPP_HAS_NO_STATIC_ASSERT

As a drive I also changed our C++03 static_assert to use _Static_assert if available.

I plan to commit this without review if nobody voices an objection.

Reviewers: mclow.lists

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D24895

llvm-svn: 282347