[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

test/PrintAsCxx/empty.swift

@@ -103,7 +103,7 @@
 // CHECK:       } // namespace swift
 // CHECK-EMPTY:
 // CHECK-NEXT:  #endif
-// CHECK:       namespace empty __attribute__((swift_private)) {
+// CHECK:       namespace empty __attribute__((swift_private)) SWIFT_SYMBOL_MODULE("empty") {
 // CHECK:       } // namespace empty
 // CHECK:       #endif