Support prefix-less `#line` preprocessor directive without file

The GNU compilers also support a line preprocessor directive with just a number without a file specified.

```
# 3
__LINE__ __FILE__
```

`gcc -E output`:
```
# 0 "a.cpp"
# 0 "<built-in>"
# 0 "<command-line>"
# 1 "/usr/include/stdc-predef.h" 1 3 4
# 0 "<command-line>" 2
# 1 "a.cpp"
# 3 "a.cpp"
3 "a.cpp"
```

actual:
```

2 "a.cpp"
```