Only emit files with subsections_via_symbols if all inputs had that set.

When generating a relocatable file, its only valid to set this flag if
all of the inputs also had the flag.  Otherwise we may atomize incorrectly
when we link the relocatable file again.

Reviewed by Lang Hames.

Differential Revision: http://reviews.llvm.org/D16018

llvm-svn: 257976