go/analysis/passes/stringintconv: make first fix behavior-preserving

Go 1.28 rejects string(x) conversions where x is a non-byte,
non-rune integer (go.dev/issue/3939). To let "go fix" migrate code
before a module's go directive is raised to go1.28, this CL makes
the analyzer's first suggested fix preserve the existing behavior,
and adds the analyzer to the go fix suite.

The old "Convert a single rune to a string" fix, string(rune(x)),
was not behavior-preserving for types wider than 32 bits due to
truncation. For such types the fix now uses fmt.Sprintf("%c", x),
which is exactly equivalent to string(x). The plain rune(x) form is
still used for constants that fit in 32 bits and for types of at
most 32 bits.

The safe fix is now offered first, since that's what go fix applies.
The decimal-formatting fix, which changes behavior, is second; it now
uses fmt.Sprintf("%d", x), for symmetry, instead of fmt.Sprint(x).
If no behavior-preserving fix is available no fixes are offered at
all, so that go fix never applies the second fix.

Also:
- simplify the edits of both Sprintf fixes by choosing the range to
  rewrite once: string(x) -> fmt.Sprintf(...), and
  mystring(x) -> mystring(fmt.Sprintf(...)).
  This fixes a bug in the old fmt.Sprint fix, which dropped the
  conversion S(x) when S was a type parameter whose sole term
  is string, yielding an ill-typed result.
- update the doc, which recommended strconv.Itoa.

Updates golang/go#81760

Change-Id: If445f650d045125475b2051e4c50e28f630a11ae
Reviewed-on: https://go-review.googlesource.com/c/tools/+/840805
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Alex Putman <aputman@golang.org>
Auto-Submit: Alan Donovan <adonovan@google.com>