Errors with the same span should only print the erroneous span once

[bstrie]

The following program:

fn main() {
    foo::bar::baz();
}

produces the following output:

blah.rs:2:4: 2:17 error: unresolved name
blah.rs:2     foo::bar::baz();
              ^~~~~~~~~~~~~
blah.rs:2:4: 2:17 error: use of undeclared module `foo::bar`
blah.rs:2     foo::bar::baz();
              ^~~~~~~~~~~~~
blah.rs:2:4: 2:17 error: unresolved name: foo::bar::baz
blah.rs:2     foo::bar::baz();
              ^~~~~~~~~~~~~
error: aborting due to 3 previous errors

How noisy! All three errors have the exact same span, so rustc should be smart enough to only print the span once, like so:

blah.rs:2:4: 2:17 error: unresolved name
                  error: use of undeclared module `foo::bar`
                  error: unresolved name: foo::bar::baz
blah.rs:2     foo::bar::baz();
              ^~~~~~~~~~~~~
error: aborting due to 3 previous errors

[recrack]

I'm working on it.

[brson]

@recrack I reverted this change because it reorders related messages in undesirable ways. There are a number of places in the compiler that emit related diagnostics in sequence and their order needs to be preserved. One thing we could do is add transactions that group any emitted diagnostics together. The order of each transaction would be preserved.