The variable declared by a TypeSwitchGuard must be
visible in each TypeCaseClause and must not conflict
with other variables declared by the initial SimpleStmt
of a type switch.

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

R=rsc
CC=golang-dev
https://golang.org/cl/4896053