coverage: Rearrange the code for embedding per-function coverage metadata
This is a series of refactorings to the code that prepares and embeds per-function coverage metadata records (“covfun records”) in the `__llvm_covfun` linker section of the final binary. The `llvm-cov` tool reads this metadata from the binary when preparing a coverage report.
Beyond general cleanup, a big motivation behind these changes is to pave the way for re-landing an updated version of #133418.
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There should be no change in compiler output, as demonstrated by the absence of (meaningful) changes to coverage tests.
The first patch is just moving code around, so I suggest looking at the other patches to see the actual changes.
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try-job: x86_64-gnu
try-job: x86_64-msvc
try-job: aarch64-apple
13c13ee4ec