Auto merge of #138766 - Zalathar:unused-fn, r=SparrowLii

coverage: Deal with unused functions and their names in one place

When coverage codegen creates dummy instances and covfun records for unused functions, we already know that they are unused, so we might as well set up the special array of unused function names at the same time.

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The first commit only moves code around; all significant changes are in the second commit.

There should be no change in compiler output.

Author: bors