Add line numbers and columns to error messages spanning multiple files

If an error message is emitted that spans several files, only the
primary file currently has line and column data attached. This is
useful information, even in files other than the one in which the error
occurs. We can often work out which line and column the error
corresponds to in other files — in this case it is helpful to add them
(in the case of ambiguity, the first relevant line/column is picked,
which is still helpful than none).

src/librustc_errors/snippet.rs

@@ -27,7 +27,8 @@ pub struct FileInfo {
 
     /// The "primary file", if any, gets a `-->` marker instead of
     /// `>>>`, and has a line-number/column printed and not just a
-    /// filename.  It appears first in the listing. It is known to
+    /// filename (other files are not guaranteed to have line numbers
+    /// or columns). It appears first in the listing. It is known to
     /// contain at least one primary span, though primary spans (which
     /// are designated with `^^^`) may also occur in other files.
     primary_span: Option<Span>,