cmd/compile, go/types: add missing space in error message

Otherwise "print(nil)" gets an error like
    use of untyped nil in argument to built-inprint

Change-Id: Iaa841df106f90787031dce75a49af69860a9dbc9
Reviewed-on: https://go-review.googlesource.com/c/go/+/611918
Commit-Queue: Ian Lance Taylor <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Robert Griesemer <[email protected]>
Auto-Submit: Ian Lance Taylor <[email protected]>
Auto-Submit: Ian Lance Taylor <[email protected]>

Author: ianlancetaylor

src/cmd/compile/internal/types2/builtins.go

@@ -641,7 +641,7 @@ func (check *Checker) builtin(x *operand, call *syntax.CallExpr, id builtinId) (
 		if nargs > 0 {
 			params = make([]Type, nargs)
 			for i, a := range args {
-				check.assignment(a, nil, "argument to built-in"+predeclaredFuncs[id].name)
+				check.assignment(a, nil, "argument to built-in "+predeclaredFuncs[id].name)
 				if a.mode == invalid {
 					return
 				}