[Parse] Refactor get/set accessor parsing

* Use 'parseAbstractFunctionBody()' for accessors as well. This
  simplifies the implementation, and makes 'parseAbstractFunctionBody()'
  the single point of parsing body of every 'AbstructFunctionDecl' types.

Author: rintaro

include/swift/Parse/Parser.h

@@ -379,25 +379,14 @@ class Parser {
 
   void restoreParserPosition(ParserPosition PP, bool enableDiagnostics = false) {
     L->restoreState(PP.LS, enableDiagnostics);
-
-    // We might be at tok::eof now, so ensure that consumeToken() does not
-    // assert about lexing past eof.
-    Tok.setKind(tok::unknown);
-    consumeTokenWithoutFeedingReceiver();
-
+    L->lex(Tok, LeadingTrivia, TrailingTrivia);
     PreviousLoc = PP.PreviousLoc;
   }
 
   void backtrackToPosition(ParserPosition PP) {
     assert(PP.isValid());
-
     L->backtrackToState(PP.LS);
-
-    // We might be at tok::eof now, so ensure that consumeToken() does not
-    // assert about lexing past eof.
-    Tok.setKind(tok::unknown);
-    consumeTokenWithoutFeedingReceiver();
-
+    L->lex(Tok, LeadingTrivia, TrailingTrivia);
     PreviousLoc = PP.PreviousLoc;
   }
 
@@ -896,8 +885,6 @@ class Parser {
 
   void consumeGetSetBody(AbstractFunctionDecl *AFD, SourceLoc LBLoc);
 
-  void parseAccessorAttributes(DeclAttributes &Attributes);
-
   struct ParsedAccessors;
   bool parseGetSetImpl(ParseDeclOptions Flags,
                        GenericParamList *GenericParams,