[ADT] Use Empty Base Optimization for Allocators

In D94439, BumpPtrAllocator changed its implementation to use an empty base optimization for the underlying allocator.
This patch builds on that by extending its functionality to more classes as well as enabling the underlying allocator to be a reference type, something not currently possible as you can't derive from a reference.

The main place this sees use is in StringMaps which often use the default MallocAllocator, yet have to pay the size of a pointer for no reason.

Reviewed By: dblaikie

Differential Revision: https://reviews.llvm.org/D129206