swift234

[proj-tree] Use a passed in BumpPtrAllocator instead of SmallVector for memory.

Previously I was using a large SmallVector to create Nodes for the
ProjectionTree. This created an issue when the SmallVector would convert
from small to large representation in the middle of a method on an
object that is stored in the SmallVector. Thus the 'this' pointer will
be invalidated and all sorts of fun times will occur.

I switched now to using a BumpPtrAllocator which is passed into the tree
and used in FunctionSignatureOptimization for all projection trees.

<rdar://problem/19534462>

Swift SVN r24706