[nfc] [lldb] Prevent needless copies of DataExtractor

lldb_private::DataExtractor contains DataBufferSP m_data_sp which is
relatively expensive to copy (due to multi-threading locking).

llvm::DataExtractor does not have this problem as it uses StringRef
instead.

The copy constructor is explicit as otherwise it is easy to make
unintended modification of a local copy instead of a caller's instance
(D107470 but that is llvm::DataExtractor).

Reviewed By: clayborg

Differential Revision: https://reviews.llvm.org/D107485