Allow use of gcc4.7's __is_literal_type for std::is_literal.

Clang's built-in is named __is_literal. GCC 4.7 added __is_literal_type which
does the same thing. Introduce _LIBCPP_IS_LITERAL() so type_traits can take
advantage of both.

Change-Id: I095ab03d93a78f2f9c6829bd81ad7413fb67eebb
_LIBCPP_IS_LITERAL already abstracts feature + version check

Change-Id: I622ec334a5da6bfd974fcb27d3dbc01f0fa3d1e5
Merge pull request #135 from awong-dev/meta.unary.prop-is_literal

Meta.unary.prop is literal