[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.

llvm-svn: 218345