[clang] Adding cfi_unchecked_callee in function type conversion suppresses diagnosis regarding other info

[brunodf-snps]

C++ test case:

void f();

// adding cfi_unchecked_callee is OK
void (*fptr1)() __attribute__((cfi_unchecked_callee)) = &f;

// adding noreturn is NOT ok
void (*fptr2)() __attribute__((noreturn)) = &f;

// BUG: adding cfi_unchecked_callee absolves adding noreturn ??
void (*fptr3)() __attribute__((cfi_unchecked_callee,noreturn)) = &f;

Godbolt demo: https://godbolt.org/z/1P4onfG7K

There was a similar issue for the conversion in C mode. This was part of a larger issue that was resolved in #160477.

[ilovepi]

CC: @PiJoules