[Allocator] Stop forward-declaring BumpPtrAllocator in a few places.
This is a necessary step to lifting some of its configuration into
template parameters rather than runtime parameters.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205140 91177308-0d34-0410-b5e6-96231b3b80d8
[Allocator] Simplify unittests by using the default size parameters in
more places.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205141 91177308-0d34-0410-b5e6-96231b3b80d8