[dev.typeparams] go/types: use a new ast.ListExpr for multi-type instances

Modify go/parser to consistently represent type instantiation as an
ast.IndexExpr, rather than use an ast.CallExpr (with Brackets:true) for
instantiations with multiple type parameters. To enable this, introduce
a new ast expr type: ListExpr.

This brings go/types in line with types2, with the exception of a small
change to funcInst to eliminate redundant errors if values are
erroneously used as types. In a subsequent CL, call.go and expr.go will
be marked as reviewed.

This also catches some type instance syntax using '()' that was
previously accepted incorrectly. Tests are updated accordingly.

Change-Id: I30cd0181c7608f1be7486a9a8b63df993b412e85
Reviewed-on: https://go-review.googlesource.com/c/go/+/293010


Trust: Robert Findley <rfindley@google.com>
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>