Improved macro handling in gcc

[Kimplul]

Here is the fix for #3320. I went with a solution where if the macro expansion error happens in the current file, set lnum and col accordingly. If the error happens in some other file, there's really no easy way to know on which line the error should be placed on in the current file, so just let lnum be 0.

For example, consider this situation:

/* c.cpp */
#include <string>
#include "header1.h"

int main(){
   return 0;
}

/* header1.h */
#include "header2.h"

/* header2.h */
std::string str = TEST;

# compile with
g++ c.cpp -DTEST='w'

The error reported by GCC would be

<command-line>: error: ‘w’ was not declared in this scope
header2.h:1:19: note: in expansion of macro ‘TEST’
    1 | std::string str = TEST;
      |                   ^~~~

To figure out on which line in c.cpp we should place the error we would need to go through the different headers in c.cpp until we find header2.h. I think that's a bit overkill, and just letting the lnum be zero with the text Error found in macro expansion. See :ALEDetail is (hopefully) good enough.

[w0rp]

Yep, this is good. We handle just this particular kind of error in this specific way, and it hopefully shouldn't break anything.

[w0rp]

Cheers! 🍻

[Kimplul]

Cheers! 🍺