Use LLVM's SetVector insteead of unordered_set.
Use LLVM's SetVector insteead of unordered_set.

SetVector guarantees ordering, so with that we can get a deterministic
output for error messages.

Differential Revision: https://reviews.llvm.org/D43254

git-svn-id: f6089bf0e6284f307027cef4f64114ee9ebb0424@325064 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/lld,llvm-mirror/lld