[trace] Use vector instead of ArrayRef when reading data

I incorrectly returned an ArrayRef when the underlying object didn't own
the data. Instead, returning a vector<uint8_t> is what we should do.

This fixes an issue when trying to access an intel-pt trace buffer
larger than 16 MB.

repro
```
go to a breakpoint
thread trace start -s 16777216
n
thread trace dump instructions # this doesn't fail anymore
```

Differential Revision: https://reviews.llvm.org/D122192