[lldb][NFC] Prevent slicing when converting DataExtractors

LLDB's implementation of DWARFDataExtractor has a method that returns a
llvm::DWARFDataExtractor. In some cases, like DebugNamesDWARFIndex::Create, we
were passing an LLVM::DWARFDataExtractor to a function that expects a
LLVM:DataExtractor by value. This is causing slicing of the derived class.

While slicing is not inherently bad, it can be dangerous if the constructor of
the derived class mutates the base class in a way that leaves it in an invalid
state after slicing.

Differential Revision: https://reviews.llvm.org/D153913