Annotate BumpPtrAllocator for MemorySanitizer.

This change adds MemorySanitizer annotations to BumpPtrAllocator to
improve report quality.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@174051 91177308-0d34-0410-b5e6-96231b3b80d8