[llvm] [ADT] Add llvm::countr_zero_constexpr (PR #164188)

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``````````bash
git-clang-format --diff origin/main HEAD --extensions h,cpp -- llvm/include/llvm/ADT/bit.h llvm/include/llvm/Support/MathExtras.h llvm/unittests/ADT/BitTest.cpp --diff_from_common_commit
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View the diff from clang-format here.
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``````````diff
diff --git a/llvm/include/llvm/ADT/bit.h b/llvm/include/llvm/ADT/bit.h
index 2fcb46a33..5971b7504 100644
--- a/llvm/include/llvm/ADT/bit.h
+++ b/llvm/include/llvm/ADT/bit.h
@@ -151,8 +151,7 @@ template <typename T, typename = std::enable_if_t<std::is_unsigned_v<T>>>
 /// Count the number of set bits in a value.
 /// Ex. popcount(0xF000F000) = 8
 /// Returns 0 if Value is zero.
-template <typename T>
-[[nodiscard]] constexpr int popcount(T Value) noexcept {
+template <typename T> [[nodiscard]] constexpr int popcount(T Value) noexcept {
   static_assert(std::is_unsigned_v<T>, "T must be an unsigned integer type");
   static_assert(sizeof(T) <= 8, "T must be 8 bytes or less");
 

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https://github.com/llvm/llvm-project/pull/164188