Exclude private decls from interface token hash.

After parsing a private decl, reset the token hash state to what it was
before the decl was parsed. This way, adding a private decl or editing
its type or name will not trigger a rebuild of downstream files.

<rdar://problem/22239821> Modifying private APIs shouldn't cause dependents to recompile

Swift SVN r31358

lib/Parse/ParseDecl.cpp

@@ -174,6 +174,36 @@ namespace {
       P.markWasHandled(D);
     }
   };
+
+  /// An RAII type to exclude tokens contributing to private decls from the
+  /// interface hash of the source file. On destruct, it checks if the set of
+  /// attributes includes the "private" attribute; if so, it resets the MD5
+  /// hash of the source file to what it was when the IgnorePrivateDeclTokens
+  /// instance was created, thus excluding from the interface hash all tokens
+  /// parsed in the meantime.
+  struct IgnorePrivateDeclTokens {
+    Parser &TheParser;
+    DeclAttributes &Attributes;
+    Optional<llvm::MD5> SavedHashState;
+
+    IgnorePrivateDeclTokens(Parser &P, DeclAttributes &Attrs)
+      : TheParser(P), Attributes(Attrs) {
+      if (TheParser.IsParsingInterfaceTokens) {
+        SavedHashState = TheParser.SF.getInterfaceHashState();
+      }
+    }
+
+    ~IgnorePrivateDeclTokens() {
+      if (!SavedHashState)
+        return;
+
+      if (auto *attr = Attributes.getAttribute<AbstractAccessibilityAttr>()) {
+        if (attr->getAccess() == Accessibility::Private) {
+          TheParser.SF.setInterfaceHashState(*SavedHashState);
+        }
+      }
+    }
+  };
 }
 
 /// \brief Main entrypoint for the parser.
@@ -1711,6 +1741,7 @@ void Parser::delayParseFromBeginningToHere(ParserPosition BeginParserPosition,
                    BeginParserPosition.PreviousLoc);
   skipUntil(tok::eof);
 }
+
 /// \brief Parse a single syntactic declaration and return a list of decl
 /// ASTs.  This can return multiple results for var decls that bind to multiple
 /// values, structs that define a struct decl and a constructor, etc.
@@ -1742,6 +1773,7 @@ ParserStatus Parser::parseDecl(SmallVectorImpl<Decl*> &Entries,
                                   StructureMarkerKind::Declaration);
 
   DeclAttributes Attributes;
+  IgnorePrivateDeclTokens IgnoreTokens(*this, Attributes);
   if (Tok.hasComment())
     Attributes.add(new (Context) RawDocCommentAttr(Tok.getCommentRange()));
   bool FoundCCTokenInAttr;