[clang] [clang] [SemaCXX] Implement CWG2627 Bit-fields and narrowing conversions (PR #78112)

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cor3ntin wrote:

Given that this is a core issue resolution, i think it should be applied in all language modes without warning, even under pedantics. This is compounding by the fact that MSVC never warned (gcc warns in all language modes - they don't seem to implement the DR yet)

(unless we have some reason to believe it would be disruptive)

https://github.com/llvm/llvm-project/pull/78112