[libc++] Remove invalid use of `#if _LIBCPP_STD_VER >= 11`, as `_LIBCPP_STD_VER` can never be less than 11.

[curdeius]

Summary

The relevant part of __config is:

#ifndef _LIBCPP_STD_VER
#  if  __cplusplus <= 201103L
#    define _LIBCPP_STD_VER 11

[curdeius]

Remove unnecessary #if altogether.

[mclow.lists]

if you need to remove something for C++03, then the test would be:
#ifndef _LIBCPP_CXX03_LANG

[curdeius]

In D93025#2445711, @mclow.lists wrote:

if you need to remove something for C++03, then the test would be:
#ifndef _LIBCPP_CXX03_LANG

That's exactly what I did in the first try. Please see diff https://reviews.llvm.org/D93025?id=310846.
But as one might have expected, there were tests already that depended on this (incorrect #if).
I'm still thinking what the best course of action should be.

[curdeius]

Actually, the choices are:

1. keep the status quo, it means that there will be an #if that is equivalent to #if 1 and is very misleading
2. remove this #if altogether (diff no. 2)
3. change to #ifndef _LIBCPP_CXX03_LANG and fix the tests that fail (that's what I'm trying to do)

[curdeius]

This has been introduced in D68480. @__simt__, what was the idea behind this #if _LIBCPP_STD_VER >= 11?
There are other parts that use __cxx_atomic_wait but are not guarded on pre-C++11. Cf. lines 1668, 1670 and 2524, 2527.

[__simt__]

I'm drawing a blank. Proceed.

libcxx/include/atomic

 };
 
 #ifdef __linux__
     using __cxx_contention_t = int32_t;
 #else
     using __cxx_contention_t = int64_t;
 #endif //__linux__
 
-#if _LIBCPP_STD_VER >= 11
+#ifndef _LIBCPP_CXX03_LANG
 
 using __cxx_atomic_contention_t = __cxx_atomic_impl<__cxx_contention_t>;
 
 #ifndef _LIBCPP_HAS_NO_PLATFORM_WAIT
 
 _LIBCPP_AVAILABILITY_SYNC _LIBCPP_EXPORTED_FROM_ABI void __cxx_atomic_notify_one(void const volatile*);
 _LIBCPP_AVAILABILITY_SYNC _LIBCPP_EXPORTED_FROM_ABI void __cxx_atomic_notify_all(void const volatile*);
 _LIBCPP_AVAILABILITY_SYNC _LIBCPP_EXPORTED_FROM_ABI __cxx_contention_t __libcpp_atomic_monitor(void const volatile*);