Try to add code coverage support for Rust projects

[alexcrichton]

Support for LLVM-based code coverage for Rust was implemented awhile
back (and has a nice blog post with documentation), and we ran
across this internally today. I'm curious if this change is all that's
needed to enable source coverage for Rust builds! I'm not 100% sure how
coverage is all set up in oss-fuzz but I figured this might be a good
place to get the ball rolling.

cc #3469

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[alexcrichton]

@googlebot I signed it!

[inferno-chromium]

There is quite a bit needed here, so Philippe started on it, see here - #4697

[alexcrichton]

Ah ok great! I'll leave it to y'all then who definitely know more about what needs to be done!