[PECOFF] Improve /export compatibility.

Looks like the rule of /export is more complicated than
I was thinking. If /export:foo, for example, is given, and
if the actual symbol name in an object file is _foo@<number>,
we need to export that symbol as foo, not as the mangled name.

If only /export:_foo@<number> is given, the symbol is exported
as _foo@<number>.

If both /export:foo and /export:_foo@<number> are given,
they are considered as duplicates, and the linker needs to
choose the unmangled name.

The basic idea seems that the linker needs to export a symbol
with the same name as given as /export.

We exported mangled symbols. This patch fixes that issue.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@223341 91177308-0d34-0410-b5e6-96231b3b80d8