General cleanup to minimize the .debug_types patch

This cleanup is designed to make the https://reviews.llvm.org/D32167 patch smaller and easier to read.

Cleanup in this patch:

Allow DWARFUnit subclasses to hand out the data that should be used when decoding data for a DIE. The information might be in .debug_info or could be in .debug_types. There is a new virtual function on DWARFUnit that each subclass must override:

virtual const lldb_private::DWARFDataExtractor &DWARFUnit::GetData() const;
This allows DWARFCompileUnit and eventually DWARFTypeUnit to hand out different data to be used when decoding the DIE information.

Add a new pure virtual function to get the size of the DWARF unit header:

virtual uint32_t DWARFUnit::GetHeaderByteSize() const = 0;
This allows DWARFCompileUnit and eventually DWARFTypeUnit to hand out different offsets where the first DIE starts when decoding DIE information from the unit.

Added a new function to DWARFDataExtractor to get the size of an offset:

size_t DWARFDataExtractor::GetDWARFSizeOfOffset() const;
Removed dead dumping and parsing code in the DWARFDebugInfo class.
Inlined a bunch of calls in DWARFUnit for accessors that were just returning integer member variables.
Renamed DWARFUnit::Size() to DWARFUnit::GetHeaderByteSize() as it clearly states what it is doing and makes more sense.

Differential Revision: https://reviews.llvm.org/D46606

llvm-svn: 331892