Replace _LIBCPP_HAS_NO_<C++03 feature> with _LIBCPP_CXX03_LANG in vector.

This patch cleans up all usages of the following feature test macros inside
<vector> and its tests:

* _LIBCPP_HAS_NO_RVALUE_REFERENCES
* _LIBCPP_HAS_NO_VARIADICS
* _LIBCPP_HAS_NO_GENERALIZED_INITIALIZERS

Where needed the above guards were replaced with _LIBCPP_CXX03_LANG.

llvm-svn: 300410