[libc] Make UInt<T> trivially copyable

[mikhail.ramalho]

Summary

This patch defaults the copy constructor and copy operator so it can be
used with __builtin_bit_cast

libc/src/__support/UInt.h

template <size_t Bits> struct UInt {
   static constexpr size_t WORDCOUNT = Bits / 64;
   uint64_t val[WORDCOUNT];
 
   static constexpr uint64_t MASK32 = 0xFFFFFFFFu;
 
   static constexpr uint64_t low(uint64_t v) { return v & MASK32; }
   static constexpr uint64_t high(uint64_t v) { return (v >> 32) & MASK32; }
 
-  constexpr UInt() {}
+  constexpr UInt() = default;
 
-  constexpr UInt(const UInt<Bits> &other) {
-    for (size_t i = 0; i < WORDCOUNT; ++i)
-      val[i] = other.val[i];
-  }
+  constexpr UInt(const UInt<Bits> &other) = default;
 
   template <size_t OtherBits> constexpr UInt(const UInt<OtherBits> &other) {
     if (OtherBits >= Bits) {
       for (size_t i = 0; i < WORDCOUNT; ++i)
         val[i] = other[i];
     } else {
       size_t i = 0;
       for (; i < OtherBits / 64; ++i)
template <size_t Bits> struct UInt {
   constexpr explicit operator uint32_t() const {
     return uint32_t(uint64_t(*this));
   }
 
   constexpr explicit operator uint8_t() const {
     return uint8_t(uint64_t(*this));
   }
 
-  UInt<Bits> &operator=(const UInt<Bits> &other) {
-    for (size_t i = 0; i < WORDCOUNT; ++i)
-      val[i] = other.val[i];
-    return *this;
-  }
+  UInt<Bits> &operator=(const UInt<Bits> &other) = default;
 
   constexpr bool is_zero() const {
     for (size_t i = 0; i < WORDCOUNT; ++i) {
       if (val[i] != 0)
         return false;
     }
     return true;
   }