Use a stricter return type in buildSectionOrder. NFC.
Use a stricter return type in buildSectionOrder. NFC.

We sort inside output sections, so all the sections we see should be
InputSectionBase.

I noticed the patch adding callgraph based section ordering used this
type and changing this separately makes the merge easier.

git-svn-id: f6089bf0e6284f307027cef4f64114ee9ebb0424@325094 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/lld,llvm-mirror/lld