ELF: Do not instantiate InputSectionBase::Discarded.

"Discarded" section is a marker for discarded sections, and we do not
use the instance except for checking its identity. In that sense, it
is just another type of a "null" pointer for InputSectionBase. So,
it doesn't have to be a real instance of InputSectionBase class.

In this patch, we no longer instantiate Discarded section but instead
use -1 as a pointer value. This eliminates a global variable which
needed initialization at startup.

llvm-svn: 261761