COFF: Suppress "Duplicate entry" warning of lib.exe

We create a module-definition file and give that to lib.exe to
create an import library file. A module-definition has to be
syntactically and semantically correct, of course.

There was a case that we created a module-definition file that
lib.exe would complain for duplicate entries. If a user gives
an unmangled and mangled name for the same symbol, we would end
up having two duplicate lines for the mangled name in a module-
definition file.

This patch fixes that issue by uniquefying entries by mangled
symbol name.

llvm-svn: 243587