[Allocator] Make the underlying allocator a template instead of an

abstract interface. The only user of this functionality is the JIT
memory manager and it is quite happy to have a custom type here. This
removes a virtual function call and a lot of unnecessary abstraction
from the common case where this is just a *very* thin vaneer around
a call to malloc.

Hopefully still no functionality changed here. =]

llvm-svn: 206149