Split InputSectionDescription::Sort into SortInner and SortOuter.

Summary:
The comparator function to compare input sections as instructed by
SORT command was a bit too complicated because it needed to handle
four different cases. This patch split it into two function calls.

This patch also simplifies the parser.

Reviewers: grimar

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D23140

llvm-svn: 277780