sync: add Map.Clear
Fixes #61696
Change-Id: I0a31afd3bc433fc84280d56f2798bda10da61eba
GitHub-Last-Rev:
17bedc864f1685178a42b59f7083677a6124f831
GitHub-Pull-Request: golang/go#61702
Reviewed-on: https://go-review.googlesource.com/c/go/+/515015
Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: qiulaidongfeng <2645477756@qq.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>