[libc] [llvm] [libc] Refactor `BigInt` (PR #86137)

[Nick Desaulniers via llvm-commits]

================
@@ -244,18 +244,30 @@ LIBC_INLINE constexpr bool is_negative(cpp::array<word, N> &array) {
 enum Direction { LEFT, RIGHT };
 
 // A bitwise shift on an array of elements.
+// TODO: Make the result UB when 'offset' is greater or equal to the number of
----------------
nickdesaulniers wrote:

Or make the case of `offset >= TOTAL_BITS` into a LIBC_ASSERT?

https://github.com/llvm/llvm-project/pull/86137