[ELF] Don't create an output section named `/DISCARD/` if it is assigned to the special phdr `NONE`

Fixes the remaining issue of PR41673 after D61186: with `/DISCARD/ { ... } :NONE`,
we may create an output section named `/DISCARD/`.

Note, if an input section is named `/DISCARD/`, ld.bfd discards it but
lld keeps it. It is probably not worth copying this behavior as it is unrealistic.

Reviewed By: ruiu

Differential Revision: https://reviews.llvm.org/D62768

llvm-svn: 362356