build: add -Wthread-safety-pointer

This will become available in Clang 21:

> ThreadSafetyAnalysis now supports -Wthread-safety-pointer, which
> enables warning on passing or returning pointers to guarded variables
> as function arguments or return value respectively. Note that
> ThreadSafetyAnalysis still does not perform alias analysis. The
> feature will be default-enabled with -Wthread-safety in a future release.

See
https://github.com/llvm/llvm-project/blob/main/clang/docs/ReleaseNotes.rst.

Author: fanquake

CMakeLists.txt

@@ -443,6 +443,7 @@ else()
   try_append_cxx_flags("-Wvla" TARGET warn_interface SKIP_LINK)
   try_append_cxx_flags("-Wshadow-field" TARGET warn_interface SKIP_LINK)
   try_append_cxx_flags("-Wthread-safety" TARGET warn_interface SKIP_LINK)
+  try_append_cxx_flags("-Wthread-safety-pointer" TARGET warn_interface SKIP_LINK)
   try_append_cxx_flags("-Wloop-analysis" TARGET warn_interface SKIP_LINK)
   try_append_cxx_flags("-Wredundant-decls" TARGET warn_interface SKIP_LINK)
   try_append_cxx_flags("-Wunused-member-function" TARGET warn_interface SKIP_LINK)