cmd/gc: reject builtin function calls in len(fixed array) constants

Fixes #7385.

LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews
https://golang.org/cl/84010044

src/cmd/gc/const.c

@@ -1629,10 +1629,25 @@ hascallchan(Node *n)
 	if(n == N)
 		return 0;
 	switch(n->op) {
+	case OAPPEND:
 	case OCALL:
 	case OCALLFUNC:
-	case OCALLMETH:
 	case OCALLINTER:
+	case OCALLMETH:
+	case OCAP:
+	case OCLOSE:
+	case OCOMPLEX:
+	case OCOPY:
+	case ODELETE:
+	case OIMAG:
+	case OLEN:
+	case OMAKE:
+	case ONEW:
+	case OPANIC:
+	case OPRINT:
+	case OPRINTN:
+	case OREAL:
+	case ORECOVER:
 	case ORECV:
 		return 1;
 	}

test/const5.go

@@ -18,6 +18,7 @@ var s [][30]int
 
 func f() *[40]int
 var c chan *[50]int
+var z complex128
 
 const (
 	n1 = len(b.a)
@@ -29,5 +30,8 @@ const (
 
 	n6 = cap(f())  // ERROR "is not a constant|is not constant"
 	n7 = cap(<-c) // ERROR "is not a constant|is not constant"
+	n8 = real(z) // ERROR "is not a constant|is not constant"
+	n9 = len([4]float64{real(z)}) // ERROR "is not a constant|is not constant"
+
 )