[ELF] With --vs-diagnostics, print a separate message for each location of a duplicate symbol.

We extract and print the source location in the message header so that
Visual Studio is able to parse it and jump there. As duplicate symbols
are defined in several locations, it is more convenient to have separate
error messages, which allows a user to easily access all the locations.

Differential Revision: https://reviews.llvm.org/D65213

llvm-svn: 367536