Flatten bool arguments under -Zflatten-format-args=yes

[KisaragiEffective]

cc #78356

The following code can be optimized:

// pub is avoiding DCE on godbolt, please ignore it
pub fn main() {
    println!("Hello World {}", false);
}

as if written as:

pub fn main() {
    println!("Hello World false");
}

compiler version: rustc 1.71.0-nightly (a2b1646c5 2023-05-25)

@rustbot label: +I-slow

[m-ou-se]

@KisaragiEffective Do you have any real world examples that would benefit from inlining boolean literals into format_args?