ELFObjectFile.h: Silence warning on Windows

The compiler was warning about using | on a uintptr_t and bool:

  Object/ELFObjectFile.h(131) : warning C4805: '|' : unsafe
  mix of type 'uintptr_t' and type 'bool' in operation

I think the warning might be useful in other cases, so I added
a cast instead of disabling it altogether.

llvm-svn: 188079