[interop][SwiftToCxx] Annotate emitted declarations with Clang's external_source_symbol attribute

Each emitted declaration is annotated with the external_source_symbol with its own USR, to allow Clang's indexer to recognize this declaration as a Swift declaration with a specific USR

lib/PrintAsClang/PrintClangFunction.cpp

@@ -905,6 +905,9 @@ ClangRepresentation DeclAndTypeClangFunctionPrinter::printFunctionSignature(
     os << " const";
   if (modifiers.isNoexcept)
     os << " noexcept";
+  if (modifiers.hasSymbolUSR)
+    ClangSyntaxPrinter(os).printSymbolUSRAttribute(
+        modifiers.symbolUSROverride ? modifiers.symbolUSROverride : FD);
   return resultingRepresentation;
 }
 
@@ -1311,6 +1314,7 @@ void DeclAndTypeClangFunctionPrinter::printCxxMethod(
       isa<FuncDecl>(FD) ? cast<FuncDecl>(FD)->isMutating() : false;
   modifiers.isConst =
       !isa<ClassDecl>(typeDeclContext) && !isMutating && !isConstructor;
+  modifiers.hasSymbolUSR = !isDefinition;
   auto result = printFunctionSignature(
       FD, signature,
       isConstructor ? getConstructorName(FD)
@@ -1375,6 +1379,8 @@ void DeclAndTypeClangFunctionPrinter::printCxxPropertyAccessorMethod(
   modifiers.isInline = true;
   modifiers.isConst =
       !isStatic && accessor->isGetter() && !isa<ClassDecl>(typeDeclContext);
+  modifiers.hasSymbolUSR = !isDefinition;
+  modifiers.symbolUSROverride = accessor->getStorage();
   auto result = printFunctionSignature(
       accessor, signature, remapPropertyName(accessor, resultTy), resultTy,
       FunctionSignatureKind::CxxInlineThunk, modifiers);