Support sancov

* [ ] (rustc) Make sure `asan` is supported on all relevant platforms (tracking)
* [ ] (cargo-cov) Build sancov-enabled binaries (easy)
* [ ] (cargo-cov) Run sancov-enabled binaries to get the `*.sancov` files (easy)
* [ ] (cov) Construct Graph by analyzing the DWARF info (very hard)

https://clang.llvm.org/docs/SanitizerCoverage.html

