[libc][NFC] fix uint128 init in float properties (#75084)

An unsigned integer was being initialized with -1 to set all its bits to
1, but this doesn't work for the BigInt class which may be used as an
integer type on systems that don't support uint128 natively. This patch
moves the initialization to use ~T(0) instead.

libc/src/__support/FPUtil/FloatProperties.h

@@ -85,7 +85,9 @@ template <typename T, size_t count> static constexpr T mask_trailing_ones() {
   static_assert(cpp::is_unsigned_v<T>);
   constexpr unsigned t_bits = CHAR_BIT * sizeof(T);
   static_assert(count <= t_bits && "Invalid bit index");
-  return count == 0 ? 0 : (T(-1) >> (t_bits - count));
+  // It's important not to initialize T with -1, since T may be BigInt which
+  // will take -1 as a uint64_t and only initialize the low 64 bits.
+  return count == 0 ? 0 : ((~T(0)) >> (t_bits - count));
 }
 
 // Derives more properties from 'FPBaseProperties' above.
@@ -131,7 +133,7 @@ struct FPCommonProperties : private FPBaseProperties<fp_type> {
   LIBC_INLINE_VAR static constexpr UIntType FP_MASK =
       mask_trailing_ones<UIntType, TOTAL_BITS>();
   static_assert((SIG_MASK & EXP_MASK & SIGN_MASK_) == 0, "masks disjoint");
-  static_assert((SIG_MASK | EXP_MASK | SIGN_MASK_) == FP_MASK, "masks covers");
+  static_assert((SIG_MASK | EXP_MASK | SIGN_MASK_) == FP_MASK, "masks cover");
 
   LIBC_INLINE static constexpr UIntType bit_at(int position) {
     return UIntType(1) << position;