[NFC] Fix the warning Wcovered-switch-default

There is a warning saying:

ASTWriter.h:766:5: error: default label in switch which covers all
enumeration values [-Werror,-Wcovered-switch-default]
  766 |     default:
      |     ^
1 error generated.

And this patch tries to fix this.

Author: ChuanqiXu9

clang/include/clang/Serialization/ASTWriter.h

@@ -763,9 +763,8 @@ class ASTWriter : public ASTDeserializationListener,
       return DeclDependentNonTemplateCXXMethodAbbrev;
     case FunctionDecl::TK_DependentFunctionTemplateSpecialization:
       return DeclDependentSpecializationCXXMethodAbbrev;
-    default:
-      llvm_unreachable("Unknwon Template Kind!");
     }
+    llvm_unreachable("Unknwon Template Kind!");
   }
   unsigned getDeclTemplateTypeParmAbbrev() const {
     return DeclTemplateTypeParmAbbrev;