[libcxx] Add deprecation notices to macros deprecated in P0883R2

When P0883R2 was initially implemented in D103769 #pragma clang deprecated didn't exist yet.
We also forgot to cleanup usages in libc++ itself.

This takes care of both.

Differential Revision: https://reviews.llvm.org/D115995

NOKEYCHECK=True
GitOrigin-RevId: 4e730aeb731cb9b7116ed51bbef662d363f72b66
diff --git a/src/barrier.cpp b/src/barrier.cpp
index bd56664..54890b2 100644
--- a/src/barrier.cpp
+++ b/src/barrier.cpp
@@ -22,7 +22,7 @@
     struct alignas(64) /* naturally-align the heap state */ __state_t
     {
         struct {
-            __atomic_base<__barrier_phase_t> __phase = ATOMIC_VAR_INIT(0);
+          __atomic_base<__barrier_phase_t> __phase{0};
         } __tickets[64];
     };