implement parsing, AST, and basic Sema support for 'defer'.

SILGen support and diagnosing jumps out of a defer aren't done
yet.



Swift SVN r27759

include/swift/Parse/Parser.h

@@ -1153,6 +1153,7 @@ public:
   ParserResult<Stmt> parseStmtBreak();
   ParserResult<Stmt> parseStmtContinue();
   ParserResult<Stmt> parseStmtReturn();
+  ParserResult<Stmt> parseStmtDefer();
   ParserStatus parseStmtCondition(StmtCondition &Result, Diag<> ID);
   ParserResult<Stmt> parseStmtIf(LabeledStmtInfo LabelInfo);
   ParserResult<Stmt> parseStmtIfConfig(BraceItemListKind Kind