[libc++] Improve LIBCXX_ENABLE_INCOMPLETE_FEATURES.

@tcanens pointed out the current behavior of the macro breaks the usage
pattern described in http://wg21.link/SD6
```
#  if __has_include(<optional>)
#    include <optional>
#    if __cpp_lib_optional >= 201606
#      define have_optional 1
#    endif
```

To support this usage pattern the hard errror is removed. Instead the
header includes nothing but the `<version>` header.

Reviewed By: ldionne, #libc

Differential Revision: https://reviews.llvm.org/D107134