[libc++][libc++abi] Build with -fsized-deallocation

This patch makes libc++ and libc++abi build with -fsized-deallocation.
That flag is enabled by default in recent versions of Clang, so this
patch will make these runtimes 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)

libcxxabi/CMakeLists.txt

@@ -247,6 +247,10 @@ endif()
 add_compile_flags("${LIBCXXABI_ADDITIONAL_COMPILE_FLAGS}")
 add_library_flags("${LIBCXXABI_ADDITIONAL_LIBRARIES}")
 
+# 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.
+add_compile_flags(-fsized-deallocation)
+
 # Configure compiler. Must happen after setting the target flags.
 include(config-ix)