Implement std::experimental::propagate_const from LFTS v2

Summary:
An implementation of std::experimental::propagate_const from Library Fundamentals Technical Specification v2.

No tests are provided for disallowed types like fancy pointers or function pointers as no code was written to handle these.

Reviewers: EricWF, mclow.lists

Differential Revision: http://reviews.llvm.org/D12486

llvm-svn: 273122

libcxx/include/experimental/__config

@@ -25,6 +25,10 @@
 #define _LIBCPP_END_NAMESPACE_LFTS  } } }
 #define _VSTD_LFTS _VSTD_EXPERIMENTAL::fundamentals_v1
 
+#define _LIBCPP_BEGIN_NAMESPACE_LFTS_V2 _LIBCPP_BEGIN_NAMESPACE_EXPERIMENTAL inline namespace fundamentals_v2 {
+#define _LIBCPP_END_NAMESPACE_LFTS_V2  } } }
+#define _VSTD_LFTS_V2 _VSTD_EXPERIMENTAL::fundamentals_v2
+
 #define _LIBCPP_BEGIN_NAMESPACE_LFTS_PMR _LIBCPP_BEGIN_NAMESPACE_LFTS namespace pmr {
 #define _LIBCPP_END_NAMESPACE_LFTS_PMR _LIBCPP_END_NAMESPACE_LFTS }
 #define _VSTD_LFTS_PMR _VSTD_LFTS::pmr