[InstrProf] Fix bug when merging empty profile with multiple threads

When merging profiles with multiple threads, the `mergeWriterContexts()` function is used to merge profile data between writers. This must be in sync with `loadInput()` which merges profiles to a single writer. This diff merges the profile kind correctly in `mergeWriterContexts()` to fix a subtle bug.

Reviewed By: phosek

Differential Revision: https://reviews.llvm.org/D139755