DebugInfo: Allow isa<> on DIDescriptor and subclasses

Allow LLVM-style casting on `DIDescriptor` and its subclasses so they
can behave more like raw pointers.  I haven't bothered with tests since
I have a follow-up commit coming shortly that uses them extensively in
tree, and I'm hoping to kill `DIDescriptor` entirely before too long (so
they won't have time to bitrot).

Usage examples:

    DIDescriptor D = foo();
    if (DICompileUnit CU = dyn_cast<MDCompileUnit>(D))
      return bar(CU);
    else if (auto *SP = dyn_cast<MDSubprogram>(D))
      return baz(SP);
    return other(D);

llvm-svn: 234250