RFC: Up front type information generation in clang

As an owner of several out-of-tree frontends, I'm mostly happy with this. It does mean I have to understand DWARF at a larger scope (fine with me, I know people). It also means I don't have to screw around in LLVM to add my platform-specific tags for things like VMS descriptors and BLISS macros as those get hoisted to my frontend(s).