[codex] Build libc++ with ASan enabled

[dzbarsky]

Author: zbarsky-bot

Summary

libcxx.static and libcxx.shared now preserve //config:asan while runtime transitions reset every other sanitizer. The generated libc++ __config_site defines _LIBCPP_INSTRUMENTED_WITH_ASAN from //config:asan_enabled, so out-of-line std::string functions update ASan container poison state. asan_libcxx_container_overflow_test covers the -fno-exceptions short-to-long std::string transition from the bazel-playground external-include-paths reproducer.

On Linux, sanitizer_common_symbolizer_internal_object replaces the previous sanitizer_common_symbolizer_internal and @llvm-project//llvm:Symbolize linkage. cc_internal_symbolizer_object matches compiler-rt/lib/sanitizer_common/symbolizer/scripts/build_symbolizer.sh when COMPILER_RT_ENABLE_INTERNAL_SYMBOLIZER=ON: it builds private libc++ and libc++abi archives in the __InternalSymbolizer namespace, builds LLVM with Zlib enabled and Zstd and threads disabled, combines the bitcode, and internalizes every definition except the seven sanitizer symbolizer APIs. //config:internal_symbolizer selects libc++ headers for LLVM Symbolize even when the caller platform selects libstdc++. sanitizer_common_symbolizer_internal_object is enabled on Bazel 9, where external_include_paths keeps the private libc++ headers before the C library headers. On Bazel 8, sanitizer runtimes omit sanitizer_common_symbolizer_internal_object, asan_output_test uses @llvm//tools:llvm-symbolizer, and asan_libcxx_container_overflow_test is incompatible. The private namespace prevents internal symbolizer definitions from replacing ASan-annotated std::__1 definitions. asan_output_test verifies an internally symbolized main frame on Linux and retains the external @llvm//tools:llvm-symbolizer on macOS.

Validation

• External Linux ASan+UBSan reproducer
• ASan container-overflow regression and sanitizer output tests
• Bazel 8 ASan container-overflow regression with a libc++ platform and with a libstdc++ caller platform
• Bazel 8.7 skips the ASan container-overflow regression and passes asan_output_test with external llvm-symbolizer
• Bazel 9.2.0rc1 Linux ARM ASan container-overflow and internal-symbolizer output tests
• Internal symbolizer object build; llvm-nm reports exactly the seven sanitizer symbolizer APIs as global definitions and no symbol outside upstream global_symbols.txt
• buildifier -mode=check on every tracked Starlark and BUILD file, and git diff --check