Remove _LIBCPP_HAS_NO_DEFAULTED_FUNCTIONS

libc++ no longer supports C++11 compilers that don't implement `= default`.
This patch removes all instances of the feature test macro
_LIBCPP_HAS_NO_DEFAULTED_FUNCTIONS as well as the potentially dead code it hides.


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@287321 91177308-0d34-0410-b5e6-96231b3b80d8