[ELF] For VS-style diagnostics, prefer printing full paths in the header.

The filename part in the message header is used by Visual Studio
to fill Error List so that a user can click on an item and jump
to the mentioned location. If we use only the name of a source file
and not the full path, Visual Studio might be unable to find the right
file or, even worse, show a wrong one.

Differential Revision: https://reviews.llvm.org/D65875

llvm-svn: 368409