[msan] Unpoison indirect outputs for userspace using memset for large operands (#79924)

Modify #77393 to clear shadow memory using `llvm.memset.*` when the size
is large, similar to `shouldUseBZeroPlusStoresToInitialize` in clang for
`-ftrivial-auto-var-init=`. The intrinsic, if lowered to libcall, will
use the msan interceptor.

The instruction selector lowers a `StoreInst` to multiple stores, not
utilizing `memset`. When the size is large (e.g.
`store { [100 x i32] } zeroinitializer, ptr %12, align 1`), the
generated code will be long (and `CodeGenPrepare::optimizeInst` will
even crash for a huge size).

```
// Test stack size
template <class T>
void DoNotOptimize(const T& var) { // deprecated by https://github.com/google/benchmark/pull/1493
  asm volatile("" : "+m"(const_cast<T&>(var)));
}

int main() {
  using LargeArray = std::array<int, 1000000>;
  auto large_stack = []() { DoNotOptimize(LargeArray()); };
  /////// CodeGenPrepare::optimizeInst triggers an assertion failure when creating an integer type with a bit width>2**23
  large_stack();
}
```