[PECOFF] Keep unmangled name in the export table descriptor

The export table descriptor is a data structure to keep information
about the export table. It contains a symbol name, and the name may
or may not be mangled.

We need unmangled names for the export table, so we demangle them
before writing them to the export table.

Obviously this is not a correct round-trip conversion. That could
drop a leading underscore from a symbol because that's
indistinguishable from a mangled name.

What we need to do is to keep unmangled names. This patch does that.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@218345 91177308-0d34-0410-b5e6-96231b3b80d8
[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