[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

lib/SymbolGraphGen/SymbolGraphGen.cpp

@@ -12,7 +12,6 @@
 
 #include "llvm/Support/JSON.h"
 #include "llvm/Support/Path.h"
-#include "swift/AST/Module.h"
 #include "swift/SymbolGraphGen/SymbolGraphGen.h"
 
 #include "SymbolGraphASTWalker.h"
@@ -22,7 +21,7 @@ using namespace symbolgraphgen;
 
 namespace {
 int serializeSymbolGraph(SymbolGraph &SG,
-                             const SymbolGraphOptions &Options) {
+                         const SymbolGraphOptions &Options) {
   SmallString<256> FileName(SG.M.getNameStr());
   if (SG.ExtendedModule.hasValue()) {
     FileName.push_back('@');