Rollup merge of #119842 - Zalathar:kind, r=oli-obk

coverage: Add enums to accommodate other kinds of coverage mappings

Extracted from  #118305.

LLVM supports several different kinds of coverage mapping regions, but currently we only ever emit ordinary “code” regions.  This PR performs the plumbing required to add other kinds of regions as enum variants, but does not add any specific variants other than `Code`.

The main motivation for this change is branch coverage, but it will also allow separate experimentation with gap regions and skipped regions, which might help in producing more accurate and useful coverage reports.

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``@rustbot`` label +A-code-coverage

Author: matthiaskrgr