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

[via cfe-commits]

https://github.com/Sirraide approved this pull request.

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, e.g.
```c
void p1(void);
void n1();
void p2(void) __attribute__((noreturn));
void n2() __attribute__((noreturn));

void (*f1)() = p1;
void (*f2)() = n1;
void (*f3)() __attribute__((noreturn)) = p1;
void (*f4)() __attribute__((noreturn)) = n1;

void (*f5)() = p2;
void (*f6)() = n2;
void (*f7)() __attribute__((noreturn)) = p2;
void (*f8)() __attribute__((noreturn)) = n2;
```

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