Refactor the code that calculates the offsets for the various
pieces of the DWARF .debug_names index section, to make it easier to
share the code with other tools, such as LLD.

Clang format fixes.

Change 'FindDebugNamesOffsets' function: make the return type 'void' (return
nothing); update the name to start with lowercase letter.  Break 'offset'
variable in NameIndex::extract into two parts: hdrSize (to be passed to
Header::extract and findDebugNamesOffsets) and 'Offset', to be used for
offsets after call to findDebugNamesOffsets.

Change comments in header file to doxygen comments.

Clang format fixes.