[XRay] Handle allocator exhaustion in segmented array

Summary:
This change allows us to handle allocator exhaustion properly in the
segmented array implementation. Before this change, we relied on the
caller of the `trim` function to provide a valid number of elements to
trim. This change allows us to do the right thing in case the elements
to trim is greater than the size of the container.

Reviewers: mboerger, eizan

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D53484

llvm-svn: 344880