[trace][intelpt] Add task timer classes

I'm adding two new classes that can be used to measure the duration of long
tasks as process and thread level, e.g. decoding, fetching data from
lldb-server, etc. In this first patch, I'm using it to measure the time it takes
to decode each thread, which is printed out with the `dump info` command. In a
later patch I'll start adding process-level tasks and I might move these
classes to the upper Trace level, instead of having them in the intel-pt
plugin. I might need to do that anyway in the future when we have to
measure HTR. For now, I want to keep the impact of this change minimal.

With it, I was able to generate the following info of a very big trace:

```
(lldb) thread trace dump info                                                                                                            Trace technology: intel-pt

thread #1: tid = 616081
  Total number of instructions: 9729366

  Memory usage:
    Raw trace size: 1024 KiB
    Total approximate memory usage (excluding raw trace): 123517.34 KiB
    Average memory usage per instruction (excluding raw trace): 13.00 bytes

  Timing:
    Decoding instructions: 1.62s

  Errors:
    Number of TSC decoding errors: 0
```

As seen above, it took 1.62 seconds to decode 9.7M instructions. This is great
news, as we don't need to do any optimization work in this area.

Differential Revision: https://reviews.llvm.org/D123357