demangler: don't treat lambda as a substitution candidate

libiberty/ChangeLog:

	PR demangler/96143
	* cp-demangle.c (d_lambda): Don't add substitution candidate.
	* testsuite/demangle-expected: Update a few existing test cases
	accordingly, and add a new test case.

libiberty/cp-demangle.c

@@ -3776,9 +3776,6 @@ d_lambda (struct d_info *di)
       ret->u.s_unary_num.num = num;
     }
 
-  if (! d_add_substitution (di, ret))
-    return NULL;
-
   return ret;
 }