This maybe used to quickly verify the code coverage of a modified test
locally (e.g. fuzzer).
Example:
```
# Build and run target; the raw profile will be written to $LLVM_PROFILE_FILE when the program exits
$ bazel build --config=dbg --config=fuzzer_asan --config=coverage //test/core/end2end/fuzzers:api_fuzzer
$ LLVM_PROFILE_FILE="api_fuzzer.profraw" bazel-bin/test/core/end2end/fuzzers/api_fuzzer test/core/end2end/fuzzers/api_fuzzer_corpus/*
# Create coverage report
$ llvm-profdata-14 merge -sparse api_fuzzer.profraw -o api_fuzzer.profdata
$ llvm-cov-14 report ./bazel-bin/test/core/end2end/fuzzers/api_fuzzer --instr-profile=api_fuzzer.profdata
```
Sample report:
f94e444f25/gistfile1.txt
One trick is that the binary needs to be statically linked, e.g. specify
`linkstatic = 1` on the BUILD target.
See https://clang.llvm.org/docs/SourceBasedCodeCoverage.html for more
info.
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Yijie Ma