Introduce mcdc::TVIdxBuilder (LLVM side, NFC) (#80676)

This is a preparation of incoming Clang changes (#82448) and just checks
`TVIdx` is calculated correctly. NFC.

`TVIdxBuilder` calculates deterministic Indices for each Condition Node.
It is used for `clang` to emit `TestVector` indices (aka ID) and for
`llvm-cov` to reconstruct `TestVectors`.

This includes the unittest `CoverageMappingTest.TVIdxBuilder`.

See also
https://discourse.llvm.org/t/rfc-coverage-new-algorithm-and-file-format-for-mc-dc/76798