Introduce extra scope for the variable declared by a
TypeSwitchGuard so that it doesn't conflict with vars
declared by the initial SimpleStmt of a type switch.

This is a replacement for CL 4896053 which caused
a build breakage.

Also:
- explicitly detect type switches (as opposed to detecting
  expression switches and then do extra testing for type switches)
- fix all outstanding TODOs in parser.go
- ran all tests

R=rsc
CC=golang-dev
https://golang.org/cl/4914044