Extracts the drive and directory parts from AFileName.
Regardless of whether AFileName is a path or filename,
this routine returns the path up to the last path.sep
eg, AFileName contains K:\Development\TypeScript\delphirtl\tests
the result is K:\Development\TypeScript\delphirtl
Extracts the drive and directory parts from AFileName. Regardless of whether AFileName is a path or filename, this routine returns the path up to the last path.sep eg, AFileName contains K:\Development\TypeScript\delphirtl\tests the result is K:\Development\TypeScript\delphirtl