General clean up of teh dwarf writer.

1. Most significant is the folding of debug information entries.  This
reduced self hosted -g SelectionDAGISel.s from 120M to 13M and pretty
close to matching .o file sizes with gcc.

2. Debug information entry printing for debugging the dwarf code.

3. Bring all the code closer to llvm standards.

4. Misc. fixes and doc clean ups.

llvm-svn: 31379