Parsing of `#line` preprocessor directive is wrong when `\\` is present in the filename argument

```c
#include

int main() {
#line 10 "not_a_\\tab"
const char *actual = __FILE__;
const char *expected = "not_a_\\tab";

return strcmp(actual, expected);
}
```
The string comparison should return 0, but it returns a non-zero value.

Start by compiling and running the supplied C reproducer, focusing on how the #line filename argument containing \\ is parsed and used for __FILE__. Trace that preprocessor path to identify the incorrect handling, then verify that the comparison returns zero and add a regression test if the existing test location is found.

Written by the indexing model from the issue text.