[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. =]

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@206149 91177308-0d34-0410-b5e6-96231b3b80d8