Tell users to use inline-8bit-counters.

[frewsxcv]

libfuzzer defaults to trace-pc-guard, but that seems to be removed now:

• https://github.com/rust-fuzz/libfuzzer/issues/66
• ~https://github.com/llvm/llvm-project/commit/c5d3d490340fd32698ef9f29b80e97a4c1e95d11#diff-32378db1cb0e8e11ecfc62eda7181acadc661910d4e3ab214babb65af95bc7e9~ meant to link to https://github.com/llvm/llvm-project/commit/62d727061053dac28447a900fce064c54d366bd6

And confusingly, their docs still mention trace-pc-guard:

• https://github.com/llvm/llvm-project/blob/master/clang/docs/SanitizerCoverage.rst