[ADT] Apply Rule of Five to StringTable::Iterator (llvm#160815)

StringTable::Iterator has a user-defined copy assignment operator, a defaulted copy constructor, and a defaulted move constructor. This patch makes the copy assignment operator defaulted and adds a defaulted move assignment operator to adhere to the Rule of Five while making the operators constexpr.

Author: kazutakahirata

llvm/include/llvm/ADT/StringTable.h

@@ -118,12 +118,8 @@ class StringTable {
  constexpr Iterator(const Iterator &RHS) = default;
  constexpr Iterator(Iterator &&RHS) = default;
 
- Iterator &operator=(const Iterator &RHS) {
- Table = RHS.Table;
- O = RHS.O;
- S = RHS.S;
- return *this;
- }
+ constexpr Iterator &operator=(const Iterator &RHS) = default;
+ constexpr Iterator &operator=(Iterator &&RHS) = default;
 
  bool operator==(const Iterator &RHS) const {
  assert(Table == RHS.Table && "Compared iterators for unrelated tables!");