[WIP] Add portable Clang safety annotations

[SchrodingerZhu]

Summary

• add portable Clang thread-safety and lifetime annotation shims
• annotate the lock-backed structures and APIs that map cleanly onto Clang's analyses
• detect and enable supported lifetime-safety warning and inference flags in CMake

Validation

• clang++ -std=c++20 -I src -mcx16 -DSNMALLOC_USE_WAIT_ON_ADDRESS=1 -fsyntax-only -Wthread-safety src/test/func/pool/pool.cc
• clang++ -std=c++20 -I src -mcx16 -DSNMALLOC_USE_WAIT_ON_ADDRESS=1 -fsyntax-only -Wthread-safety src/test/func/first_operation/first_operation.cc
• clang++ -std=c++20 -I src -mcx16 -DSNMALLOC_USE_WAIT_ON_ADDRESS=1 -fsyntax-only -Wexperimental-lifetime-safety-permissive -Wexperimental-lifetime-safety-suggestions src/test/func/pool/pool.cc
• clang++ -std=c++20 -I src -mcx16 -DSNMALLOC_USE_WAIT_ON_ADDRESS=1 -fsyntax-only -Wexperimental-lifetime-safety-permissive -Wexperimental-lifetime-safety-suggestions src/test/func/first_operation/first_operation.cc