[libc++] Change _LIBCPP_CONSTEXPR_SINCE_XXX to constexpr in the dylib

Since we build the dylib with C++20, there's no need to use conditional
macros anymore.

Differential Revision: https://reviews.llvm.org/D157995
diff --git a/libcxx/src/memory_resource.cpp b/libcxx/src/memory_resource.cpp
index 3a80b84..3786ec5 100644
--- a/libcxx/src/memory_resource.cpp
+++ b/libcxx/src/memory_resource.cpp
@@ -74,7 +74,7 @@
     __null_memory_resource_imp null_res;
   } resources;
   char dummy;
-  _LIBCPP_CONSTEXPR_SINCE_CXX14 ResourceInitHelper() : resources() {}
+  constexpr ResourceInitHelper() : resources() {}
   ~ResourceInitHelper() {}
 };