[flang] Add return statement to Logical opeator=.

This removes the current undefined behavior where the function has a return type
but no return statement.

Fixes: https://bugs.llvm.org/show_bug.cgi?id=45530

flang/include/flang/Evaluate/logical.h

@@ -29,6 +29,7 @@ template <int BITS, bool IS_LIKE_C = true> class Logical {
 
   template <int B, bool C> constexpr Logical &operator=(Logical<B, C> x) {
     word_ = Represent(x.IsTrue());
+    return *this;
   }
 
   // Fortran actually has only .EQV. & .NEQV. relational operations