ADT: Sink the guts of StringMapEntry::Create into StringMapEntryBase

Sink the interesting parts of StringMapEntry::Create into a new function
StringMapEntryBase::allocateWithKey that's only templated on the
allocator, taking the entry size and alignment as parameters.

As dblaikie pointed out in the review, it'd be interesting as a
follow-up to make this more generic and maybe sink at least some of it
into a source file; I haven't done that yet myself, but I left behind an
encouraging comment.

Differential Revision: https://reviews.llvm.org/D95654