DebugInfo: Overload get() in DIDescriptor subclasses

Continue to simplify the `DIDescriptor` subclasses, so that they behave
more like raw pointers.  Remove `getRaw()`, replace it with an
overloaded `get()`, and overload the arrow and cast operators.  Two
testcases started to crash on the arrow operators with this change
because of `scope:` references that weren't real scopes.  I fixed them.
Soon I'll add verifier checks for them too.

This also adds explicit dereference operators.  Previously, the builtin
dereference against `operator MDNode *()` would have worked, but now the
builtins are ambiguous.

llvm-svn: 233030