coverage: Optionally instrument the RHS of lazy logical operators
(This is an updated version of #124644 and #124402. Fixes #124120.)
When `||` or `&&` is used outside of a branching context (such as the condition of an `if`), the rightmost value does not directly influence any branching decision, so branch coverage instrumentation does not treat it as its own true-or-false branch.
That is a correct and useful interpretation of “branch coverage”, but might be undesirable in some contexts, as described at #124120. This PR therefore adds a new coverage level `-Zcoverage-options=condition` that behaves like branch coverage, but also adds additional branch instrumentation to the right-hand-side of lazy boolean operators.
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As discussed at https://github.com/rust-lang/rust/issues/124120#issuecomment-2092394586, this is mainly intended as an intermediate step towards fully-featured MC/DC instrumentation. It's likely that we'll eventually want to remove this coverage level (rather than stabilize it), either because it has been incorporated into MC/DC instrumentation, or because it's getting in the way of future MC/DC work. The main appeal of landing it now is so that work on tracking conditions can proceed concurrently with other MC/DC-related work.
````@rustbot```` label +A-code-coverage
7667a91778