Fix `format_args` capture for macro expanded format strings
Since #100996 `format_args` capture for macro expanded strings aren't prevented when the span of the expansion points to a string literal, e.g.
```rust
// not a terribly realistic example, but also happens for proc_macros that set
// the span of the output to an input str literal, such as indoc
macro_rules! x {
($e:expr) => { $e }
}
fn main() {
let a = 1;
println!(x!("{a}"));
}
```
The tests didn't catch it as the span of `concat!()` points to the macro invocation
r? `@m-ou-se`
edadc7ccdd