[llvm-branch-commits] [flang] [flang][OpenMP] Track reachable metadirective replacements (PR #219014)

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MattPD wrote:

The description still attributes the shared OpenMP matching and scoring changes to this PR, in its three bullets and the two paragraphs after them. Those changes are now in https://github.com/llvm/llvm-project/pull/224431. This PR's diff against that PR touches only Flang semantics, `bbc`, one lowering TODO, and tests. The squash message would therefore claim changes absent from the commit. Could those passages name 224431 as the prerequisite instead?

The description also omits four behavior changes in this PR:
- lowering now rejects `nothing apply(...)` with a "not yet implemented" error instead of silently dropping the transformation
- an invalid negative score is treated as absent during recovery
- every enclosing leaf directive now occupies a scoring position
- `bbc` now sets the semantics target triple and features

https://github.com/llvm/llvm-project/pull/219014