[FEATURE] Support user-supplied `#` directives with library filtering

**Describe the bug**
```
# 1 "toto.c"

int f(){return 0;}
```
This will be entirely filtered out by the 'library filter'. This is most certainly correct (as this is really 'toto.i' referencing 'toto.c'), but this is also what is produced by using `-save-temps` with GCC.

Maybe the filter could guess that if the first directive points to .c file at line 1, then this may be the main file?

I've spent some time this morning wondering which optimization decided to completely empty the result. Now that I know this is the filter not playing well with GCC preprocessed files that's ok, but I guess some people may have the same issue. Using `-P` when creating the preprocessed file can also help.

**Link**

https://godbolt.org/z/nocdcY