[PECOFF] Fix exported symbols in an import library.

Looks like if you have symbol foo in a module-definition file
(.def file), and if the actual symbol name to match that export
description is _foo@x (where x is an integer), the exported
symbol name becomes this.

  - foo in the .dll file
  - foo@x in the .lib file

I have checked in a few fixes recently for exported symbol name mangling.
I haven't found a simple rule that governs all the mangling rules.
There may not ever exist. For now, this is a patch to improve .lib
file compatibility.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@223524 91177308-0d34-0410-b5e6-96231b3b80d8