[libc++] Build with -fsized-deallocation

This patch makes libc++ build with -fsized-deallocation. That flag is
enabled by default in recent versions of Clang, so this patch will make
libc++ forward-compatible with ToT Clang.

libcxx/CMakeLists.txt

@@ -553,6 +553,10 @@ function(cxx_add_basic_build_flags target)
     target_add_compile_flags_if_supported(${target} PRIVATE -fvisibility=hidden)
   endif()
 
+  # Build with -fsized-deallocation, which is default in recent versions of Clang.
+  # TODO(LLVM 21): This can be dropped once we only support Clang >= 19.
+  target_compile_options(${target} PRIVATE -fsized-deallocation)
+
   # Let the library headers know they are currently being used to build the
   # library.
   target_compile_definitions(${target} PRIVATE -D_LIBCPP_BUILDING_LIBRARY)