[trace] clear any existing tracing sessions before relaunching the binary

There's a bug caused when a process is relaunched: the target, which
doesn't change, keeps the Trace object from the previous process, which
is already defunct, and causes segmentation faults when it's attempted
to be used.
A fix is to clean up the Trace object when the target is disposing of
the previous process during relaunches.

A way to reproduce this:
```
lldb a.out
b main
r
process trace start
c
r
process trace start
```

Differential Revision: https://reviews.llvm.org/D122176

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