Revert "Revert "Only restrict order if both sections are in the script.""

This reverts commit r282021, bringing back r282015.

The problem was that the comparison function was not a strict weak
ordering anymore, which this patch fixes.

Original message:

Only restrict order if both sections are in the script.

This matches gold and bfd behavior and is required to handle some scripts.

The script has to assume where PT_LOADs start in order to align that
spot. If we don't allow section it doesn't know about to move to the
middle, we can need more PT_LOADs and those will not be aligned.

llvm-svn: 282035