[LLD] Fix findOrphanPos to consistently ignore "dead" OutputSection's

When findOrphanPos does the reverse search to find the OutputSection
preceding the orphan's insertion point, look for a live OutputSection
and ignore "dead" OutputSection's. This matches the behaviour of the
forward search performed earlier in this function.

Added test which without the above fix fails as a result of an orphan
executable section being incorrectly placed in a non-executable segment.

Differential Review: https://reviews.llvm.org/D38690

llvm-svn: 315292