[libcxx] Remove _LIBCPP_HAS_NO_BUILTIN_IS_CONSTANT_EVALUATED

All supported compilers should support
_LIBCPP_HAS_NO_BUILTIN_IS_CONSTANT_EVALUATED so this can be removed.

Reviewed By: ldionne, #libc, Quuxplusone

Differential Revision: https://reviews.llvm.org/D107239

NOKEYCHECK=True
GitOrigin-RevId: 1123100a16a321d70508e2508ebc5d57ce7163dc
diff --git a/include/version b/include/version
index a0ec730..6adea75 100644
--- a/include/version
+++ b/include/version
@@ -328,9 +328,7 @@
 #   define __cpp_lib_integer_comparison_functions       202002L
 # endif
 # define __cpp_lib_interpolate                          201902L
-# if !defined(_LIBCPP_HAS_NO_BUILTIN_IS_CONSTANT_EVALUATED)
-#   define __cpp_lib_is_constant_evaluated              201811L
-# endif
+# define __cpp_lib_is_constant_evaluated                201811L
 // # define __cpp_lib_is_layout_compatible                 201907L
 # define __cpp_lib_is_nothrow_convertible               201806L
 // # define __cpp_lib_is_pointer_interconvertible          201907L