Re-land "[XRay] Move-only Allocator, FunctionCallTrie, and Array"

Re-land "[XRay] Move-only Allocator, FunctionCallTrie, and Array"

This reverts commit r348455, with some additional changes:

- Work-around deficiency of gcc-4.8 by duplicating the implementation of
  `AppendEmplace` in `Append`, but instead of using brace-init for the
  copy construction, use a placement new explicitly calling the copy
  constructor.

llvm-svn: 348563