cmd/compile/internal/types2: catch unexpected expression lists

This is a modified port of the https://golang.org/cl/313909
change for go/types.

- add catch-all cases for unexpected expression lists
- add Checker.singleIndex function to check single indices
- better syntax error handling in parser for invalid type
  instantiations that are missing a type argument

Change-Id: I6f0f396d637ad66b79f803d886fdc20ee55a98b3
Reviewed-on: https://go-review.googlesource.com/c/go/+/314409
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

src/cmd/compile/internal/syntax/parser.go

@@ -1291,16 +1291,15 @@ func (p *parser) typeInstance(typ Expr) Expr {
 
 	pos := p.pos()
 	p.want(_Lbrack)
-	if p.tok == _Rbrack {
-		p.error("expecting type")
-		p.next()
-		return typ
-	}
-
 	x := new(IndexExpr)
 	x.pos = pos
 	x.X = typ
-	x.Index, _ = p.typeList()
+	if p.tok == _Rbrack {
+		p.syntaxError("expecting type")
+		x.Index = p.badExpr()
+	} else {
+		x.Index, _ = p.typeList()
+	}
 	p.want(_Rbrack)
 	return x
 }