cmd/compile: add clear(x) builtin

cmd/compile: add clear(x) builtin

To clear map, and zero content of slice.

Updates #56351

Change-Id: I5f81dfbc465500f5acadaf2c6beb9b5f0d2c4045
Reviewed-on: https://go-review.googlesource.com/c/go/+/453395
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Auto-Submit: Cuong Manh Le <cuong.manhle.vn@gmail.com>