D66494 introduced the GWP-ASan stack_trace_compressor_fuzzer. Building fuzz
targets in compiler-rt is a new affair, and has some challenges:
- If the host compiler doesn't have compiler-rt, the -fsanitize=fuzzer may not be able to link against libclang_rt.fuzzer*.
- Things in compiler-rt generally aren't built when you want to build with sanitizers using -DLLVM_USE_SANITIZER. This tricky to work around, so we create the new tools directory so that we can build fuzz targets with sanitizers. This has the added bonus of fixing the problem above as well, as we can now just guard the fuzz target build to only be done with -DLLVM_USE_SANITIZE_COVERAGE=On.