SmallVector currently uses 32bit integers for size and capacity to reduce
sizeof(SmallVector). This limits the number of elements to UINT32_MAX.
For a SmallVector<char>, this limits the SmallVector size to only 4GB.
Buffering bitcode output uses SmallVector<char>, but needs >4GB output.
This changes SmallVector size and capacity to conditionally use word-size
integers if the element type is small (<4 bytes). For larger elements types,
the vector size can reach ~16GB with 32bit size.
Making this conditional on the element type provides both the smaller
sizeof(SmallVector) for larger types which are unlikely to grow so large,
and supports larger capacities for smaller element types.