[trace][intel pt] Measure the time it takes to decode a thread in per-cpu mode

This metric was missing. We were only measuring in per-thread mode, and
this completes the work.

For a sample trace I have, the `dump info` command shows

```
Timing for this thread:
    Decoding instructions: 0.12s

```

I also improved a bit the TaskTime function so that callers don't need to
specify the template argument

Differential Revision: https://reviews.llvm.org/D129249