Use llvm::StringSet instead of std::set.
Use llvm::StringSet instead of std::set.

std::set is pretty slow. We generally prefer llvm::StringSet if we don't
need an sorted set.

Differential Revision: https://reviews.llvm.org/D40579

git-svn-id: f6089bf0e6284f307027cef4f64114ee9ebb0424@319371 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/lld,llvm-mirror/lld