Allow error comments above the error site.

[Jarcho]

Clippy has some lints which take comments into account and where the error span encompasses multiple lines. Currently this requires putting the matcher many lines later and then counting the lines. e.g.

if foo {
  foo;
  bar;
  baz;
} else {
  foo;
  bar;
  baz;
} 
//~^^^^^^^^^ ERROR: same if else bodies

This patch allows putting the matcher above, avoiding the problem.

//~v ERROR: same if else bodies
if foo {
  foo;
  bar;
  baz;
} else {
  foo;
  bar;
  baz;
} 

[oli-obk]

neat!