Add a file magic for CL.exe's object file created with /GL.

This patch makes it possible to identify object files created by CL.exe
with /GL option. Such file contains Microsoft proprietary intermediate
code instead of target machine code to do LTO.

I need this to print out user-friendly error message from LLD.

Differential Revision: https://reviews.llvm.org/D26645

llvm-svn: 286919