[lldb][NFCI] Apply IndexEntry to DWARFUnitHeader outside of extraction

I plan on replacing LLDB's DWARFUnitHeader implementation with LLVM's.
LLVM's DWARFUnitHeader::extract applies the DWARFUnitIndex::Entry to a
given DWARFUnitHeader outside of the extraction because the index entry
is only relevant to one place where we may parse DWARFUnitHeaders
(specifically when we're creating a DWARFUnit in a DWO context). To ease
the transition, I've reshaped LLDB's implementation to look closer to
LLVM's.

Reviewed By: aprantl, fdeazeve

Differential Revision: https://reviews.llvm.org/D151919