This change stabilizes the result of Sort when the error list contains
multiple items for same position. To stabilize the result, newly also
the Msg field is considered.

The motivation is to avoid diffs of sorted scanner.ErrorList output
in repository tracked logs like:

-testdata/foo.go:19:44: "bar"
 testdata/foo.go:19:44: "qux"
+testdata/foo.go:19:44: "bar"

The change was approved at [0] before submitting.

As a side effect, one file in go/parser/testdata must be updated as
well. For this file the parser produces two different errors:

testdata/issue3106.src:22:5: expected ';', found 'if'
testdata/issue3106.src:22:5: expected operand, found 'if'

Before comparing the actual and expected errors, the former are
filtered to keep only one error per source line[1]. With the new
(*ErrorList).Less the outcome is the other error than before which is
kept after the call to RemoveMultiplies.

[0]: https://groups.google.com/d/msg/golang-nuts/5ChC0XiIwlU/rol_yb2gTj4J
[1]:
https://github.com/golang/go/blob/9d0239771a2ddd77be0ba64c2782e1328a378190/src/go/parser/error_test.go#L160

Change-Id: Ib72c98a891cdeef34705c22dfbeb0408dcdfddf8
Reviewed-on: https://go-review.googlesource.com/8340Reviewed-by: Robert Griesemer <gri@golang.org>