Correct #line directive to cpp2 source file

[filipsajdak]

Closes #56

After introducing output option the #line directives points to the output file path instead of cpp2 source path.

Cpp2 source file

i : int = 42;

Steps

1. build/external/cppfront external/tests/easy.cpp2 -o build/easy.cpp

build/easy.cpp content

// ----- Cpp2 support -----
#include "cpp2util.h"


#line 1 "build/easy.cpp2"

//=== Cpp2 definitions ==========================================================

#line 1 "build/easy.cpp2"
    int i { 42 }; 

Error & expected behaviour

#line directive points to file build/easy.cpp2 that does not exists. The expected path is external/tests/easy.cpp2

[hsutter]

Thanks! I saw this issue via email and did the fix, then noticed this was a PR and your fix was essentially identical to what I did. :) But please check, thanks.