[Coverage] Introduce the type `CounterPair` for RegionCounterMap. NFC. (#112724)

`CounterPair` can hold `<uint32_t, uint32_t>` instead of current
`unsigned`, to hold also the counter number of SkipPath. For now, this
change provides the skeleton and only `CounterPair::Executed` is used.

Each counter number can have `None` to suppress emitting counter
increment. 2nd element `Skipped` is initialized as `None` by default,
since most `Stmt*` don't have a pair of counters.

This change also provides stubs for the verifier. I'll provide the impl
of verifier for `+Asserts` later.

`markStmtAsUsed(bool, Stmt*)` may be used to inform that other side
counter may not emitted.

`markStmtMaybeUsed(S)` may be used for the `Stmt` and its inner will be
excluded for emission in the case of skipping by constant folding. I put
it into places where I found.

`verifyCounterMap()` will check the coverage map and the counter map,
and can be used to report inconsistency.

These verifier methods shall be eliminated in `-Asserts`.


https://discourse.llvm.org/t/rfc-integrating-singlebytecoverage-with-branch-coverage/82492

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