go/types: fix bad printf verbs

This fixes the following vet warnings:

go/types/builtins.go:437: arg call for printf verb %s of wrong type: *go/ast.CallExpr
go/types/builtins.go:598: arg call for printf verb %s of wrong type: *go/ast.CallExpr

Updates #11041

Change-Id: I746d054e8e49b330fbdf961912a98f55dd5f3ff9
Reviewed-on: https://go-review.googlesource.com/26997
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/go/types/builtins.go

@@ -434,7 +434,7 @@ func (check *Checker) builtin(x *operand, call *ast.CallExpr, id builtinId) (_ b
 			return
 		}
 		if nargs < min || min+1 < nargs {
-			check.errorf(call.Pos(), "%s expects %d or %d arguments; found %d", call, min, min+1, nargs)
+			check.errorf(call.Pos(), "%v expects %d or %d arguments; found %d", call, min, min+1, nargs)
 			return
 		}
 		var sizes []int64 // constant integer arguments, if any
@@ -595,7 +595,7 @@ func (check *Checker) builtin(x *operand, call *ast.CallExpr, id builtinId) (_ b
 			return
 		}
 		if !constant.BoolVal(x.val) {
-			check.errorf(call.Pos(), "%s failed", call)
+			check.errorf(call.Pos(), "%v failed", call)
 			// compile-time assertion failure - safe to continue
 		}
 		// result is constant - no need to record signature