don't print a span from DUMMY_SP

[pnkfelix]

(imported from improperly closed bug #22222)

Errors like:

../../../src/librustc_driver/lib.rs:1:1: 1:1 error: multiple dylib candidates for `std` found
../../../src/librustc_driver/lib.rs:1 // Copyright 2014 The Rust Project Developers. See the COPYRIGHT
                                      ^
../../../src/librustc_driver/lib.rs:1:1: 1:1 note: candidate #1: /home/julian/rust/rust/x86_64-unknown-linux-gnu/stage0/lib/rustlib/x86_64-unknown-linux-gnu/lib/libstd-4e7c5e5c.so
../../../src/librustc_driver/lib.rs:1 // Copyright 2014 The Rust Project Developers. See the COPYRIGHT
                                      ^
../../../src/librustc_driver/lib.rs:1:1: 1:1 note: candidate #2: /home/julian/rust/rust/x86_64-unknown-linux-gnu/stage1/lib/libstd-4e7c5e5c.so
../../../src/librustc_driver/lib.rs:1 // Copyright 2014 The Rust Project Developers. See the COPYRIGHT
                                      ^

are frustrating. What purpose is the span serving there -- at most it's going to, what, put someone into the correct root directory? We might be better off not printing spans at all in these cases.

[brson]

Lot's changed.