Swift: Make ExtensionDecl.toString robust to when there's more than one extended thing.

Author: geoffw0

swift/ql/lib/codeql/swift/elements/decl/ExtensionDecl.qll

@@ -2,9 +2,10 @@ private import codeql.swift.generated.decl.ExtensionDecl
 
 class ExtensionDecl extends Generated::ExtensionDecl {
   override string toString() {
-    result = "extension of " + this.getExtendedTypeDecl().toString()
+    result =
+      "extension of " + unique(NominalTypeDecl td | td = this.getExtendedTypeDecl()).toString()
     or
-    not exists(this.getExtendedTypeDecl()) and
+    count(this.getExtendedTypeDecl()) != 1 and
     result = "extension"
   }
 }