Unassign sections if they are "assigned" to /DISCARD/.

"/DISCARD/" is a special section name in the linker script to discard
input sections. Previously, we handled it as if it were a real section,
so an input section can be assigned but dead. However, allowing sections
to be

 - assigned and alive (will be emitted),
 - not assigned and dead (removed), or
 - assigned but dead (???)

feels logically wrong and practically error-prone. This patch removes
the last combination of the states.

llvm-svn: 316622