[libcxx] [llvm] [libc++] Implement P1885R12: `<text_encoding>` (PR #141312)

[Louis Dionne via llvm-commits]

================
@@ -70,8 +72,10 @@
 
 // clang-format off
 
+#  define _LIBCPP_INTRODUCED_IN_LLVM_23 0
+#  define _LIBCPP_INTRODUCED_IN_LLVM_23_ATTRIBUTE __attribute__((unavailable))
+
 // LLVM 22
-// TODO: Fill this in
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ldionne wrote:

This should not be removed. We still need to fill this in for Apple.

https://github.com/llvm/llvm-project/pull/141312