Fix unicode-character filename bug.

Issued here https://github.com/Valloric/YouCompleteMe/issues/1069
To reproduce:
1. Create any file with unicode character on its filename, e.g `Ω.c`
2. Open the file and start adding codes until >5 lines (i.e ycm start compiling)
3. YCM will crash due to unicode encoding error. See issue referenced above for complete log.

I have signed the Google CLA.