[IPO][SampleContextTracker] Use SmallVector to track context profiles to prevent non-determinism.

Use SmallVector instead of SmallSet to track the context profiles mapped. Doing this
can help avoid non-determinism caused by iterating over unordered containers.

This bug was found with reverse iteration turning on,
--extra-llvm-cmake-variables="-DLLVM_REVERSE_ITERATION=ON".
Failing LLVM test profile-context-tracker-debug.ll .

Reviewed By: MaskRay, wenlei

Differential Revision: https://reviews.llvm.org/D99547