Rollup merge of #134208 - Zalathar:covmap-covfun, r=compiler-errors

coverage: Tidy up creation of covmap and covfun records

This is a small follow-up to #134163 that mostly just inlines and renames some variables, and adds a few comments.

It also slightly defers the creation of the LLVM value that holds the filename table, to just before the value is needed.

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try-job: x86_64-mingw-2
try-job: dist-x86_64-linux