cmd/gofmt: fix computation of function header size

Function sizes are computed to determine whether a function
can be kept on one line or should be split to several lines. Part of the
computation is the function header from the FUNC token and until the
opening { token.

Prior to this change, the function header size used distance from the
original source position of the current token, which led to issues when
the source between FUNC and the original source position was rewritten
(such as whitespace being collapsed). Now we take the current output
position into account, so that header size represents the reformatted
source rather than the original source.

The following files in the Go repository are reformatted with this
change:

* strings/strings_test.go
* cmd/compile/internal/gc/fmt.go

In both cases the reformatting is minor and seems to be correct given
the heuristic to single-line functions longer than 100 columns to
multiple lines.

Fixes #28082

Change-Id: Ib737f6933e09b79e83715211421d5262b366ec93
Reviewed-on: https://go-review.googlesource.com/c/go/+/188818


Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
Reviewed-by: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>