[libc] [llvm] [libc] Remove UB specializations of type traits for `BigInt` (PR #84035)

[Nick Desaulniers via llvm-commits]

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@@ -208,8 +221,8 @@ template <typename T, typename Fmt = radix::Dec> class IntegerToString {
 
   // An internal stateless structure that handles the number formatting logic.
   struct IntegerWriter {
-    static_assert(cpp::is_integral_v<T>);
-    using UNSIGNED_T = cpp::make_unsigned_t<T>;
+    static_assert(cpp::is_integral_v<T> || cpp::is_big_int_v<T>);
+    using UNSIGNED_T = typename details::IntegerWriterUnsigned<T>::type;
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nickdesaulniers wrote:

interesting, I've not seen `typename` outside of a `template <typename T>`.  What are the language rules for when this is necessary?

https://github.com/llvm/llvm-project/pull/84035