Re-land r348335 "[XRay] Move-only Allocator, FunctionCallTrie, and Array"

Re-land r348335 "[XRay] Move-only Allocator, FunctionCallTrie, and Array"

Continuation of D54989.

Additional changes:

  - Use `.AppendEmplace(...)` instead of `.Append(Type{...})` to appease
    GCC 4.8 with confusion on when an initializer_list is used as
    opposed to a temporary aggregate initialized object.

llvm-svn: 348438