[lldb][NFCI] DecodedThread::TraceItemStorage::error should own its own data

The way it works now, it stores a `const char *` that it does not
explicitly own. It's owned by the ConstString StringPool. This is purely
to manage its lifetime, we don't really benefit from deduplication (nor
should we try to, they are errors). We also don't really benefit from
quick comparisons.

This may make the size of TraceItemStorage larger, but you have to pay
the cost of owning the data somewhere. The ConstString StringPool is an
attractive choice but ultimately a poor one.

Differential Revision: https://reviews.llvm.org/D152326