[ADT][NFC] Use empty base optimisation in BumpPtrAllocatorImpl

Most uses of this class just use the default MallocAllocator.
As this contains no fields, we can use the empty base optimisation for BumpPtrAllocatorImpl and save 8 bytes of padding for most use cases.

This prevents using a class that is marked as `final` as the `AllocatorT` template argument.
In one must use an allocator that has been marked as `final`, the simplest way around this is a proxy class.
The class should have all the methods that `AllocaterBase` expects and should forward the calls to your own allocator instance.

Reviewed By: dblaikie

Differential Revision: https://reviews.llvm.org/D94439