[ELF] - Cleanup of processSectionCommands().

The way we handle ONLY_IF_RO/ONLY_IF_RW constraints in
processSectionCommands is a bit tricky. If input sections
does no satisfy given constraint we remove command from
commands list. It seems too complex, what we can do instead
is to make the OutputCommand empty. So that at later steps
LLD will remove it just like it deal with other empty output commands.
That allows to simplify the loop a bit.

Differential revision: https://reviews.llvm.org/D39094

llvm-svn: 317082