coverage: Rearrange the code for embedding per-function coverage metadata

[Zalathar]

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.

---

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.

---

try-job: x86_64-gnu
try-job: x86_64-msvc
try-job: aarch64-apple