swift-module-digester: refactor the ABI/API diagnostics to use DiagnosticEngine. NFC (#18957)

Before this patch, we used to define tooling-specific diagnostics. With adding more
checking logics, we found this mechanism hard to extend. This patch
eliminates the home-made diagnostics model to use the one from compiler, which provides
several benefits: less boiler-templates, better integration with DiagnosticConsumer,
and easy ordering of detected issues.

tools/swift-api-digester/ModuleAnalyzerNodes.cpp

@@ -293,6 +293,22 @@ SDKNode *SDKNodeRoot::getInstance(SDKContext &Ctx) {
   return Info.createSDKNode(SDKNodeKind::Root);
 }
 
+StringRef SDKNodeDecl::getScreenInfo() const {
+  auto ModuleName = getModuleName();
+  auto HeaderName = getHeaderName();
+  auto &Ctx = getSDKContext();
+  llvm::SmallString<64> SS;
+  llvm::raw_svector_ostream OS(SS);
+  if (Ctx.getOpts().PrintModule)
+    OS << ModuleName;
+  if (!HeaderName.empty())
+    OS << "(" << HeaderName << ")";
+  if (!OS.str().empty())
+    OS << ": ";
+  OS << getDeclKind() << " " << getFullyQualifiedName();
+  return Ctx.buffer(OS.str());
+}
+
 bool SDKNodeDecl::isSDKPrivate() const {
   if (getName().startswith("__"))
     return true;