Turn a config macro (_LIBCPP_HAS_NO_INLINE_VARIABLES) into an attribute macro (_LIBCPP_INLINE_VAR)

[mclow.lists]

Summary

We're going to be defining a lot of inline variables going forward, and the current way of doing it is ... long-winded.
Add an attribute macro which we can put everywhere we need, and handles the conditional.

Instead of writing

#ifndef _LIBCPP_HAS_NO_INLINE_VARIABLES
inline
#endif
constexpr in_place_t in_place{};

we can now write:

_LIBCPP_INLINE_VAR constexpr in_place_t in_place{};

(and it gets rid of a FIXME!)

[mclow.lists]

There are no tests because this should not change any functionality.

[mclow.lists]

This was committed as part of r321658

include/__config

 #define _LIBCPP_NODISCARD_AFTER_CXX17
 #endif
 
-// FIXME: Remove all usages of this macro once compilers catch up.
-#if !defined(__cpp_inline_variables) || (__cpp_inline_variables < 201606L)
-# define _LIBCPP_HAS_NO_INLINE_VARIABLES
+#if _LIBCPP_STD_VER > 14 && defined(__cpp_inline_variables) && (__cpp_inline_variables >= 201606L)
+# define _LIBCPP_INLINE_VAR  inline
+#else
+# define _LIBCPP_INLINE_VAR  
 #endif
 
 #ifdef _LIBCPP_HAS_NO_RVALUE_REFERENCES

include/utility

 struct _LIBCPP_TYPE_VIS in_place_t {
     explicit in_place_t() = default;
 };
-#ifndef _LIBCPP_HAS_NO_INLINE_VARIABLES
-inline
-#endif
-constexpr in_place_t in_place{};
+_LIBCPP_INLINE_VAR constexpr in_place_t in_place{};
 
 template <class _Tp>
 struct _LIBCPP_TEMPLATE_VIS in_place_type_t {

     explicit in_place_type_t() = default;
 };
 template <class _Tp>
-#ifndef _LIBCPP_HAS_NO_INLINE_VARIABLES
-inline
-#endif
-constexpr in_place_type_t<_Tp> in_place_type{};
+_LIBCPP_INLINE_VAR constexpr in_place_type_t<_Tp> in_place_type{};
 
 template <size_t _Idx>
 struct _LIBCPP_TYPE_VIS in_place_index_t {

     explicit in_place_index_t() = default;
 };
 template <size_t _Idx>
-#ifndef _LIBCPP_HAS_NO_INLINE_VARIABLES
-inline
-#endif
-constexpr in_place_index_t<_Idx> in_place_index{};
+_LIBCPP_INLINE_VAR constexpr in_place_index_t<_Idx> in_place_index{};
 
 template <class _Tp> struct __is_inplace_type_imp : false_type {};
 template <class _Tp> struct __is_inplace_type_imp<in_place_type_t<_Tp>> : true_type {};