[trace] Replace TraceCursorUP with TraceCursorSP

[trace] Replace TraceCursorUP with TraceCursorSP

The use of `std::unique_ptr` with `TraceCursor` adds unnecessary complexity to adding `SBTraceCursor` bindings
Specifically, since `TraceCursor` is an abstract class there's no clean
way to provide "deep clone" semantics for `TraceCursorUP` short of
creating a pure virtual `clone()` method (afaict).

After discussing with @wallace, we decided there is no strong reason to
favor wrapping `TraceCursor` with `std::unique_ptr` over `std::shared_ptr`, thus this diff
replaces all usages of `std::unique_ptr<TraceCursor>` with `std::shared_ptr<TraceCursor>`.

This sets the stage for future diffs to introduce `SBTraceCursor`
bindings in a more clean fashion.

Test Plan:

Differential Revision: https://reviews.llvm.org/D130925