[PATCH] BumpPtr allocate SimpleReferences

[kledzik at apple.com]

A downside to BumpVector is that as the vector grows (by adding References), the capacity is doubled by bump allocating a new chunk.  All the previous allocations (e.g. if current at 8 elements, the allocs for 1, 2, and 4) are wasted space in the bumpptr pool.  

A linked list does not have this problem because the elements never move and are never copying.  They just have a pointer to the next one.

http://reviews.llvm.org/D6518