Add a new set of helper classes for creating and reading debug

information.  This logically replaces the "Desc" classes in
MachineModuleInfo.  Nice features of these classes are that they:

1. Are much more efficient than MMI because they don't create a 
   temporary parallel data structure for debug info that has to be
   'serialized' and 'deserialized' into/out of the module.
2. These provide a much cleaner abstraction for debug info than 
   MMI, which will make it easier to change the implementation in 
   the future (to be MDNode-based).
3. These are much easier to use than the MMI interfaces, requiring
   a lot less code in the front-ends.
4. These can be used to both create (for frontends) and read (for
   codegen) debug information.  DebugInfoBuilder can only be used
   to create the nodes.

So far, this is implemented just enough to support the debug info
generation needs of clang.  This can and should be extended to 
support the full set of debug info constructs, and we should switch
llvm-gcc and llc over to using this in the near future.

This code also has a ton of FIXMEs in it, because the way we 
currently represent debug info in LLVM IR is basically insane in a
variety of details.  This sort of issue should be fixed when we
eventually reimplement debug info on top of MDNodes.

llvm-svn: 58954