time: fix receiver for Time.IsDST method

Only methods that modify the time take pointer receivers;
IsDST does not modify it and therefore should not.

For #42102 and #46688.

Change-Id: I4721ef7f4d7572236ae6e4d99a459b9ffb11999e
Reviewed-on: https://go-review.googlesource.com/c/go/+/326789
Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

api/go1.17.txt

@@ -153,7 +153,7 @@ pkg time, const Layout = "01/02 03:04:05PM '06 -0700"
 pkg time, const Layout ideal-string
 pkg time, func UnixMicro(int64) Time
 pkg time, func UnixMilli(int64) Time
-pkg time, method (*Time) IsDST() bool
+pkg time, method (Time) IsDST() bool
 pkg time, method (Time) GoString() string
 pkg time, method (Time) UnixMicro() int64
 pkg time, method (Time) UnixMilli() int64

src/time/time.go

@@ -1340,7 +1340,7 @@ func UnixMicro(usec int64) Time {
 }
 
 // IsDST reports whether the time in the configured location is in Daylight Savings Time.
-func (t *Time) IsDST() bool {
+func (t Time) IsDST() bool {
  _, _, _, _, isDST := t.loc.lookup(t.Unix())
  return isDST
 }