coverage: Emit mappings for unused functions without generating stubs
For a while I've been annoyed by the fact that generating coverage maps for unused functions involves generating a stub function at the LLVM level.
As I suspected, generating that stub function isn't actually necessary, as long as we specifically tell LLVM about the symbol names of all the functions that have coverage mappings but weren't codegenned (due to being unused).
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There is some helper code that gets moved around in the follow-up patches, so look at the first patch to see the most important functional changes.
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`@rustbot` label +A-code-coverage
0d1664674a