[PECOFF] Resolve dllexported symbols.

We did not actively try to resolve dllexported symbols specified
by /export or by a module definition file. So if exported symbols
would be resolved for other reasons, like other symbols refer to
them, that was fine, but if (unreferenced) exported symbols were
in an archive file, and no one refers to that file in the archive,
they remained unresolved.

That would obviously cause the issue that dllexported symbols are
not in a resultant DLL.

In this patch, we create an undefined symbol for each dllexported
symbol, to let the core linker to resolve it.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@208452 91177308-0d34-0410-b5e6-96231b3b80d8