-Zmacro-backtrace
For macros that are implemented on the compiler, we do *not* mention the `-Zmacro-backtrace` flag. This includes `derive`s and standard macros.
When the template string passed to asm!() is produced by a macro call like concat!() we were producing wrong error spans. Now in the case of a macro call we just use the entire arg to asm!(), macro call and all, as the error span.