Introduce _LIBCPP_STD_VER.  This can be set by the client (or the clang driver).  Or it will be defaulted.  The default is 11 if -std= c++11 or eariler, else it will default to the current year modulo the century.  We anticipate it defaulting to 14 for C++14 when the time comes.  For now, post-C++11 libcxx implementations should protect themselves with #if _LIBCPP_STD_VER > 11.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@181347 91177308-0d34-0410-b5e6-96231b3b80d8