[SymbolGraph] Emit synthesized members

Emit copies of default implementations in protocol extensions and superclass declarations in conforming types and subclasses respectively using a virtual USR, i.e. `${REAL_USR}::SYNTHESIZED::${CONFORMING_OR_SUBCLASS_TYPE_USR}`.

- Add a -skip-synthesized-members option to skip these synthesized members.

- Create a new wrapping `Symbol` type that can also contain a base type declaration as well as the inherited declaration for those synthesized cases. Move some symbol-specific APIs there.

- Doc comments can “cascade” down to protocol extensions or refinements in concrete types. When emitting the doc comment for a symbol, look up through to superclasses or protocol requirements for where a doc comment is actually written.

- Clean up filtering of implicitly private (e.g. “public underscored”) types

rdar://problem/59128787

include/swift/SymbolGraphGen/SymbolGraphOptions.h

@@ -0,0 +1,43 @@
+//===--- SymbolGraphOptions.h - Swift SymbolGraph Options -----------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2017 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/ADT/Triple.h"
+#include "swift/AST/AttrKind.h"
+
+#ifndef SWIFT_SYMBOLGRAPHGEN_SYMBOLGRAPHOPTIONS_H
+#define SWIFT_SYMBOLGRAPHGEN_SYMBOLGRAPHOPTIONS_H
+
+namespace swift {
+namespace symbolgraphgen {
+
+struct SymbolGraphOptions {
+  /// The directory to output the symbol graph JSON files.
+  StringRef OutputDir;
+
+  /// The target of the module.
+  llvm::Triple Target; 
+  /// Pretty-print the JSON with newlines and indentation.
+  bool PrettyPrint;
+
+  /// The minimum access level that symbols must have in order to be
+  /// included in the graph.
+  AccessLevel MinimumAccessLevel;
+
+  /// Emit members gotten through class inheritance or protocol default
+  /// implementations with compound, "SYNTHESIZED" USRs.
+  bool EmitSynthesizedMembers;
+};
+
+} // end namespace symbolgraphgen
+} // end namespace swift
+
+#endif // SWIFT_SYMBOLGRAPHGEN_SYMBOLGRAPHOPTIONS_H