[DWARF] Have HashedNameToDIE store a DataExtractor by value

Summary:
The DataExtractors are cheap to copy so there is no reason to store them
by reference. Also, in my upcoming indexing refactor I am planning to
remove the apple tables data extractor members from the SymbolFileDWARF
class, so there will not be a DataExtractor with a suitable lifetime to
refer to.

Reviewers: clayborg, JDevlieghere

Subscribers: aprantl, lldb-commits

Differential Revision: https://reviews.llvm.org/D46888

llvm-svn: 332596