[clang] [clang] Report narrowing conversions with const references (PR #75332)

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alexfh wrote:

The change in the diagnostic happens to make it much noisier than it was before. Our code was clean w.r.t. -Wc++11-narrowing, but now we see hundreds if not thousands of compilation breakages (we use -Werror, which turns out to be the only robust way to avoid backsliding). I think, the new behavior (though useful) needs to be guarded by a separate warning flag.

https://github.com/llvm/llvm-project/pull/75332