swift-api-digester: describing external type declarations as extensions in error messages

External type declarations are synthesized to incorporate members in extensions to types
of external modules. In diagnostics, we should use 'extension' instead of 'struct/class'
for these decls to avoid confusion.

Author: nkcsgexi

test/api-digester/Inputs/cake1.swift

@@ -168,3 +168,7 @@ public protocol HasMutatingMethodClone: HasMutatingMethod {
   mutating func foo()
   var bar: Int { mutating get }
 }
+
+public extension Int {
+  public func IntEnhancer() {}
+}

test/api-digester/Outputs/Cake-abi.txt

@@ -6,6 +6,7 @@ cake1: Protocol P3 has generic signature change from <τ_0_0 : cake.P1, τ_0_0 :
 /* RawRepresentable Changes */
 
 /* Removed Decls */
+Swift: Extension Int has been removed
 cake1: Accessor GlobalVarChangedToLet.Set() has been removed
 cake1: Accessor RemoveSetters.Value.Set() has been removed
 cake1: AssociatedType RequiementChanges.removedType has been removed

test/api-digester/Outputs/Cake.txt

@@ -6,6 +6,7 @@ cake1: Protocol P3 has generic signature change from <Self : cake.P1, Self : cak
 /* RawRepresentable Changes */
 
 /* Removed Decls */
+Swift: Extension Int has been removed
 cake1: Accessor GlobalVarChangedToLet.Set() has been removed
 cake1: Accessor RemoveSetters.Value.Set() has been removed
 cake1: AssociatedType RequiementChanges.removedType has been removed

tools/swift-api-digester/ModuleAnalyzerNodes.cpp

@@ -385,7 +385,15 @@ StringRef SDKNodeDecl::getScreenInfo() const {
     OS << "(" << HeaderName << ")";
   if (!OS.str().empty())
     OS << ": ";
-  OS << getDeclKind() << " " << getFullyQualifiedName();
+  bool IsExtension = false;
+  if (auto *TD = dyn_cast<SDKNodeDeclType>(this)) {
+    IsExtension = TD->isExternal();
+  }
+  if (IsExtension)
+    OS << "Extension";
+  else
+    OS << getDeclKind();
+  OS << " " << getFullyQualifiedName();
   return Ctx.buffer(OS.str());
 }