[PECOFF] Change export table type.

This patch changes the type of export table set from std::set to
std::vector. The new code is slightly inefficient, but because
export table elements are actually mutable, std::vector is better
here. No functionality change.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@218343 91177308-0d34-0410-b5e6-96231b3b80d8