[InstrProf][Temporal] Add weight field to traces

As discussed in [0], add a `weight` field to temporal profiling traces found in profiles. This allows users to use the `--weighted-input=` flag in the `llvm-profdata merge` command to weight traces from different scenarios differently.

Note that this is a breaking change, but since [1] landed very recently and there is no way to "use" this trace data, there should be no users of this feature. We believe it is acceptable to land this change without bumping the profile format version.

[0] https://reviews.llvm.org/D147812#4259507
[1] https://reviews.llvm.org/D147287

Reviewed By: snehasish

Differential Revision: https://reviews.llvm.org/D148150