DebugInfo: Require valid DIDescriptors

As part of PR22777, switch from `dyn_cast_or_null<>` to `cast<>` in most
`DIDescriptor` accessors.  These classes are lightweight wrappers around
pointers, so the users should check for valid pointers before using
them.

This survives a Darwin clang -g bootstrap (after fixing testcases), but
it's possible the bots will complain about other configurations.  I'll
fix any fallout as quickly as I can!  Once this bakes for a bit I'll
remove the macros.

Note that `DebugLoc` implicitly gets stricter with this change as well,
since it forward to `DILocation`.  Any code that's using `DebugLoc`
accessors should check `DebugLoc::isUnknown()` first.  (BTW, I'm also
partway through a cleanup of the `DebugLoc` API to make it more obvious
what it is (a glorified pointer wrapper) and remove cruft from before
the Metadata/Value split.  I'll commit soon.)

llvm-svn: 232844