[dev.typeparams] go/types: import typexpr.go from dev.go2go

Changes from dev.go2go (compare with patchset 1):
 + Update stale comments.
 + Fix a bug in structType where check.atEnd closed over the loop
   variable, resulting in incorrect error positions.
 + Fix a bug in the CallExpr clause of typInternal where it didn't check
   e.Brackets before checking instantiatedType.
 + Remove support for parenthesized embedded type names.
 + Add an IndexExpr clause to embeddedFieldIdent.
 + Lift the substMap construction out of the loop in funcType when
   substituting receiver type parameters.
 + Minor simplification in collectTypeConstraints.

Compare with patchset 1 to see these changes.

Change-Id: I24f10e8615a0bbcd56c86ecf3490ce6a99cfebd6
Reviewed-on: https://go-review.googlesource.com/c/go/+/278916


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