[Clang] Fix -Wunused-parameter for implicit coroutine uses (#217518) Clang's coroutine semantic analysis builds references to coroutine parameters while looking up a class-specific allocation function. When overload resolution falls back to a size-only `operator new`, these speculative references currently suppress `-Wunused-parameter`. Preserve each parameter's referenced state while collecting placement arguments, then mark the parameters referenced only when those arguments are included in the selected allocation call. Keep this distinction when placement arguments are replaced by `std::nothrow`. Apply the same rule to promise initialization: when initialization using the coroutine parameter copies succeeds, mark the original parameters referenced; when it falls back to default initialization, leave them unchanged. Add regression coverage for both the matching and fallback allocation-function and promise-initialization cases, and add a release note. Testing: - `clang/test/SemaCXX/warn-unused-parameters-coroutine.cpp`: 1/1 passed. - `clang/test/SemaCXX/coroutine*.cpp`: 24/24 passed. Fixes #217501. Assisted-by: OpenAI Codex (cherry picked from commit 01fec9890ad8a4980beb8ec1945320e126ddbdcd)
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