`include_str!(file!())` does not work on godbolt

as found by @liamnaddell in #1085

#![feature(rustc_attrs)]

#[rustc_builtin_macro]
macro_rules! include_str {
    () => {{}};
}

#[rustc_builtin_macro]
macro_rules! file {
    () => {{}};
}

fn main() {
    let _ = include_str!(file!());
}

I expected to see this happen: no error

Instead, this happened: gccrs complains that it is not able to open the file at /app/<source> even though printing the result of the file!() macro shows /app/example.rs

Start by reproducing the inline Rust snippet on godbolt and compare the paths produced by file!() with the path used by include_str!(file!()). Trace the relevant gccrs macro expansion and file lookup handling. Done means the example compiles without the /app/<source> file-open error while preserving the expected file!() result.