go/parser: slightly improved error message by adding hint

It's difficult to make this much better w/o much
more effort. This is a rare case and probably not
worth it.

Fixes #6052.

R=golang-codereviews, bradfitz, adonovan
CC=golang-codereviews
https://golang.org/cl/49740045

src/pkg/go/parser/parser.go

@@ -1752,7 +1752,7 @@ func (p *parser) makeExpr(s ast.Stmt) ast.Expr {
 	if es, isExpr := s.(*ast.ExprStmt); isExpr {
 		return p.checkExpr(es.X)
 	}
-	p.error(s.Pos(), "expected condition, found simple statement")
+	p.error(s.Pos(), "expected condition, found simple statement (missing parentheses around composite literal?)")
 	return &ast.BadExpr{From: s.Pos(), To: s.End()}
 }