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

[via llvm-commits]

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@@ -43,13 +43,20 @@ struct BigInt {
   static_assert(is_integral_v<WordType> && is_unsigned_v<WordType>,
                 "WordType must be unsigned integer.");
 
+  using word_type = WordType;
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lntue wrote:

Will making these type aliases `word_t`, `unsigned_t`, and `signed_t` conflict with anything?  B/c writing them all out makes them look like variable names.

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