[PECOFF] Keep renamed undefined symbol name in export descriptor

Exported symbol name resolution is two-pass. In the first pass,
we try to resolve that as a regular undefined symbol. If it fails,
we look for mangled name for the symbol and rename the undefined
symbol and try again.

After all name resolution is done, we look for an atom for each
exported symbol again, to construct the export table. In this
process we try the regular names first, and then try mangled names.
But at this moment we should have knew which name is correct.

This patch is to keep the information we get in the first process
to use it later.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@218354 91177308-0d34-0410-b5e6-96231b3b80d8