[trace][intelpt] Support system-wide tracing [22] - Some final touches

Having a member variable TraceIntelPT * makes it look as if it was
optional. I'm using instead a weak_ptr to indicate that it's not
optional and the object is under the ownership of TraceIntelPT.

Besides that, I've simplified the Perf aux and data buffers copying by
using vector.insert.

I'm also renaming Lookup2 to Lookup. The 2 in the name is confusing.

Differential Revision: https://reviews.llvm.org/D127881