It's been reported that when using __attribute__((format)) on non-variadic
functions, certain values that normally get promoted when passed as variadic
arguments now unconditionally emit a diagnostic:
```c
void foo(const char *fmt, float f) __attribute__((format(printf, 1, 2)));
void bar(void) {
foo("%g", 123.f);
// ^ format specifies type 'double' but the argument has type 'float'
}
```
This is normally not an issue because float values get promoted to doubles when
passed as variadic arguments, but needless to say, variadic argument promotion
does not apply to non-variadic arguments.
While this can be fixed by adjusting the prototype of `foo`, this is sometimes
undesirable in C (for instance, if `foo` is ABI). In C++, using variadic
templates, this might instead require call-site fixing, which is tedious and
arguably needless work:
```c++
template<typename... Args>
void foo(const char *fmt, Args &&...args) __attribute__((format(printf, 1, 2)));
void bar(void) {
foo("%g", 123.f);
// ^ format specifies type 'double' but the argument has type 'float'
}
```
To address this issue, we teach FormatString about a few promotions that have
always been around but that have never been exercised in the direction that
FormatString checks for:
* `char`, `unsigned char` -> `int`, `unsigned`
* `half`, `float16`, `float` -> `double`
This addresses issue https://github.com/llvm/llvm-project/issues/59824.
