[XCOFF] add C_FILE symbol at index 0 of symbol table.

This is for XCOFF DWARF support.

Seems when DWARF debug is enable, symbol 0 has special usage
for AIX binder. At least, symbol 0 can not be the .text
section. Otherwise, we get some binding time error.

Add correct C_FILE symbol at index 0 here to make AIX binder
work.

Reviewed By: hubert.reinterpretcast

Differential Revision: https://reviews.llvm.org/D97117