[libcxx] [llvm] [libc++] Get the GCC build mostly clean of warnings (PR #96604)

[Louis Dionne via llvm-commits]

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@@ -18,7 +18,7 @@
 #define ATOMIC_FLAG_INIT {false}
 #define ATOMIC_VAR_INIT(__v) {__v}
 
-#if _LIBCPP_STD_VER >= 20 && !defined(_LIBCPP_DISABLE_DEPRECATION_WARNINGS)
+#if _LIBCPP_STD_VER >= 20 && defined(_LIBCPP_COMPILER_CLANG_BASED) && !defined(_LIBCPP_DISABLE_DEPRECATION_WARNINGS)
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ldionne wrote:

Do we need this if we have `-Wno-deprecated-declarations` below?

https://github.com/llvm/llvm-project/pull/96604