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"