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

[Bruno De Fraine via cfe-commits]

brunodf-snps wrote:

> LGTM, but can you add a few more tests that mix function (pointers) w/ and w/o a prototype where the arguments / return type are compatible

Thanks, I have integrated your suggestions into the last part of the test file. [Just in case the behavior is not clear: the cases `f3` and `f4` unsafely add noreturn information. Case `f4` is already diagnosed today, but case `f3` is missed, and here we rectify this.]

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