[clang] [clang][Sema] Fix incomplete C mode incompatible ExtInfo/ExtProtoInfo conversion diagnostic (PR #160477)

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https://github.com/Sirraide commented:

I’m feeling somewhat conflicted about this approach: on the one hand it’s better than what we were doing before, but on the other rebuilding a bunch of types to circumvent our own analysis... a bit weird. I wonder if we could instead extract the parts of `IsFunctionConversion()` that we care about here into a separate function.

https://github.com/llvm/llvm-project/pull/160477