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

[Bruno De Fraine via cfe-commits]

brunodf-snps wrote:

@Sirraide If there are no further remarks from your end, could you merge this PR? (No commit access here.)

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