[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
temporary commit

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@198103 91177308-0d34-0410-b5e6-96231b3b80d8
[PECOFF] Warn only when /export options are not compatible.

Currently LLD always print a warning message if the same symbol is specified
more than once for /export option. It's a bit annoying because specifying the
same symbol with compatible options is actually safe and considered as a
normal use case. This patch makes LLD to warn only when incompatible export
options are given.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@198104 91177308-0d34-0410-b5e6-96231b3b80d8