[PATCH] [libcxx] Take advantage of gcc4.7 frontend support in type_traits

[Albert Wong]

Hi mclow.lists,

Removes _LIBCPP_HAS_TYPE_TRAITS. Insteads, for each needed compiler
frontend built-in, uses a combination of __has_feature() and _GNUC_VER
to enable or disable the particular type_trait library function.

These tests now pass in gcc-4.7
  meta.unary.prop/is_trivial.pass.cpp
  meta.unary.prop/is_standard_layout.pass.cpp
  meta.unary.prop/is_nothrow_move_constructible.pass.cpp
  meta.unary.prop/is_nothrow_copy_constructible.pass.cpp
  meta.unary.prop/is_nothrow_assignable.pass.cpp
  meta.unary.prop/is_empty.pass.cpp

http://reviews.llvm.org/D4340

Files:
  include/type_traits
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