DebugInfo: Factor out RETURN_FROM_RAW, etc.

Remove the helper macros `RETURN_FROM_RAW()`,
`RETURN_DESCRIPTOR_FROM_RAW()`, and `RETURN_REF_FROM_RAW()`, since they
don't do anything too special anymore.  This loses an `assert(get())`,
but I'm hoping any crashes were shaken out when r232844 landed a few
weeks ago.

llvm-svn: 233660