[LLD][COFF] Support GNU style == aliases

D46245 added support for this in llvm-libtool, but while lld-link can
also create .lib files from .def files it didn't support aliases.

I compared the Inputs/library.def test against the output from
llvm-libtool and it matches, except for the fact that lld-link reorders
functions for some reason.

I have also verified that this fixes a bug I was running into while
trying to compile .def files to .lib files in MinGW-w64 (using lld-link
instead of llvm-libtool).

Differential Revision: https://reviews.llvm.org/D113365