[llvm-debuginfo-analyzer][NFC] Rename LVElfReader.cpp[h] (#85530)

As part of the WebAssembly support work review
  https://github.com/llvm/llvm-project/pull/82588

It was decided to rename:

  Files: LVElfReader.cpp[h] -> LVDWARFReader.cpp[h]
         ELFReaderTest.cpp  -> DWARFReaderTest.cpp

  Class: LVELFReader        -> LVDWARFReader

The name LVDWARFReader would match the another reader LVCodeViewReader
as they will reflect the type of
debug information format that they are parsing.