For expressions containing a TypeSwitchGuard with a short variable
declaration e.g. var := x.(type), the spec says that var is declared
at the beginning of the implicit block for each in each clause.
Previously, var was declared in the block for the switch statement
and each implicit block, which led to errors if the type case clause
referenced a type with a similar name as the declared variable.

Fixes golang/go#10047.

From-SVN: r221230