This is the last major patch to implement support for the .loc directive

This is the last major patch to implement support for the .loc directive
and output the dwarf line number tables.  This contains the code to emit and
encode the dwarf line tables from the previously gathered information in the
MCLineSection objects.  This contains all the details to encode the line and
address deltas into the dwarf line table.

To do this an MCDwarfLineAddrFragment has been added.

Also this moves the interface code out of Mach-O streamer into
MCDwarf so it should be useable by other object file formats.

There is now one call to be made from an MCObjectStreamer
EmitInstruction() method:
   MCLineEntry::Make(this, getCurrentSection());
to create a line entry after each instruction is assembled.

And one call call to be made from an MCObjectStreamer Finish() method:
   MCDwarfFileTable::Emit(this, DwarfLineSection);
when getContext().hasDwarfFiles() is true and is passed a object file specific
MCSection where to emit the dwarf file and the line tables.

This appears to now be correct for 32-bit targets, at least x86.  But the
relocation entries for 64-bit Darwin needs some further work which is next
up to work on.  So for now the 64-bit Mach-O target does not output the
dwarf file and line tables.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@115157 91177308-0d34-0410-b5e6-96231b3b80d8