cmd/gc: say 'non-constant array bound' instead of 'invalid array bound'

Fixes #8196.

LGTM=adonovan
R=adonovan
CC=golang-codereviews
https://golang.org/cl/141510044

src/cmd/gc/typecheck.c

@@ -408,6 +408,9 @@ reswitch:
 				v = toint(l->val);
 				break;
 			default:
+				if(l->type != T && isint[l->type->etype] && l->op != OLITERAL)
+					yyerror("non-constant array bound %N", l);
+				else
 					yyerror("invalid array bound %N", l);
 				goto error;
 			}

test/fixedbugs/bug255.go

@@ -12,4 +12,9 @@ var c [1.5]int	// ERROR "truncated"
 var d ["abc"]int   // ERROR "invalid array bound|not numeric"
 var e [nil]int     // ERROR "invalid array bound|not numeric"
 var f [e]int       // ERROR "invalid array bound|not constant"
-var g [1<<65]int	// ERROR "array bound is too large|overflows"
+var g [1 << 65]int // ERROR "array bound is too large|overflows"
+var h [len(a)]int  // ok
+
+func ff() string
+
+var i [len([1]string{ff()})]int // ERROR "non-constant array bound|not constant"