[PECOFF] Use std::set to store export symbol descriptors.

Each export symbol descriptor has unique name attribute, so std::set is
better container than std::vector for it. No functionality change.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@198102 91177308-0d34-0410-b5e6-96231b3b80d8