folly: ubsan: reduce vector size to avoid UBSAN timeout
Summary:
Based on diff where this was introduced in {
D360195} it seems like
```
// This value should we multiple of word size.
static size_t const kHeapifyCapacitySize = sizeof(
typename std::aligned_storage<
sizeof(InternalSizeType),
alignof(value_type)
>::type);
// Threshold to control capacity heapifying.
static size_t const kHeapifyCapacityThreshold =
100 * kHeapifyCapacitySize;
```
So anything above 100*sizeof(SizeType) should do.
Reviewed By: philippv
Differential Revision:
D2871422
fb-gh-sync-id:
a69e47286c53887ac05e89dab565b9d609e183a0