[libc][support][UInt] implement 128b popcount

We could have used this in the implementation of UInt::has_single_bit, but
has_single_bit has a perhaps useful early return.

Author: nickdesaulniers

libc/src/__support/UInt.h

@@ -1082,6 +1082,17 @@ bit_cast(const UInt<Bits> &from) {
   return cpp::bit_cast<To>(from.val);
 }
 
+// Specialization of cpp::popcount ('bit.h') for BigInt.
+template <typename T>
+[[nodiscard]] LIBC_INLINE constexpr cpp::enable_if_t<is_big_int_v<T>, int>
+popcount(T value) {
+  int bits = 0;
+  for (auto word : value.val)
+    if (word)
+      bits += popcount(word);
+  return bits;
+}
+
 // Specialization of cpp::has_single_bit ('bit.h') for BigInt.
 template <typename T>
 [[nodiscard]] LIBC_INLINE constexpr cpp::enable_if_t<is_big_int_v<T>, bool>

libc/test/src/__support/CPP/bit_test.cpp

@@ -15,13 +15,6 @@
 
 namespace LIBC_NAMESPACE::cpp {
 
-using UnsignedTypesNoBigInt = testing::TypeList<
-#if defined(LIBC_TYPES_HAS_INT128)
-    __uint128_t,
-#endif // LIBC_TYPES_HAS_INT128
-    unsigned char, unsigned short, unsigned int, unsigned long,
-    unsigned long long>;
-
 using UnsignedTypes = testing::TypeList<
 #if defined(LIBC_TYPES_HAS_INT128)
     __uint128_t,
@@ -228,7 +221,7 @@ TEST(LlvmLibcBitTest, Rotr) {
             rotr<uint64_t>(0x12345678deadbeefULL, -19));
 }
 
-TYPED_TEST(LlvmLibcBitTest, CountOnes, UnsignedTypesNoBigInt) {
+TYPED_TEST(LlvmLibcBitTest, CountOnes, UnsignedTypes) {
   EXPECT_EQ(popcount(T(0)), 0);
   for (int i = 0; i != cpp::numeric_limits<T>::digits; ++i)
     EXPECT_EQ(popcount<T>(cpp::numeric_limits<T>::max() >> i),