compiler: implement clear builtin for slices

testdata/go1.21.go

@@ -1,6 +1,7 @@
 package main
 
 func main() {
+	// The new min/max builtins.
 	ia := 1
 	ib := 5
 	ic := -3
@@ -9,4 +10,9 @@ func main() {
 	fc := -3.0
 	println("min/max:", min(ia, ib, ic), max(ia, ib, ic))
 	println("min/max:", min(fa, fb, fc), max(fa, fb, fc))
+
+	// The clear builtin, for slices.
+	s := []int{1, 2, 3, 4, 5}
+	clear(s[:3])
+	println("cleared s[:3]:", s[0], s[1], s[2], s[3], s[4])
 }

testdata/go1.21.txt

@@ -1,2 +1,3 @@
 min/max: -3 5
 min/max: -3.000000e+000 +5.000000e+000
+cleared s[:3]: 0 0 0 4 5