Change ExplodedNode to have its NodeGroups all BumpPtrAllocated, avoiding...

Change ExplodedNode to have its NodeGroups all BumpPtrAllocated, avoiding malloc() traffic when adding successors/predecessors to a node.  This was done by introducing BumpVector, which is essentially SmallVector with all memory being BumpPtrAllocated (this can certainly be cleaned up or moved into llvm/ADT).

This change yields a 1.8% speed increase when running the analyzer (with -analyzer-store=region) on a small benchmark file.

llvm-svn: 83439