cmd/gofmt: normalize integer imaginary literals starting with 0

An 'i' suffix on an integer literal marks the integer literal as
a decimal integer imaginary value, even if the literal without the
suffix starts with a 0 and thus looks like an octal value:

	0123i == 123i // != 0123 * 1i

This is at best confusing, and at worst a potential source of bugs.
It is always safe to rewrite such literals into the equivalent
literal without the leading 0.

This CL implements this normalization.

Change-Id: Ib77ad535f98b5be912ecbdec20ca1b472c1b4973
Reviewed-on: https://go-review.googlesource.com/c/162538


Run-TryBot: Robert Griesemer <gri@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>