swift234

Remove SemiStmt class

We have no intention of ever supporting actual semicolon statements
(separators, statements no), nor do we ever want to because that would
mean the behavior of the program would potentially change if semicolons
were naively removed.

This patch tracks the trailing semicolon now in the decl/expr/stmt,
which will enable someone to write a good "swift indent" tool in the
future.

Swift SVN r3824