Reapply r150631:

    "Add a completed/incomplete type difference. This allows us to have
    partial types for contexts and forward decls while allowing us to
    complete types later on for debug purposes.

    This piggy-backs on the metadata replacement and rauw changes
    for temporary nodes and takes advantage of the incremental
    support I added in earlier. This allows us to, if we decide,
    to limit adding methods and variables to structures in order
    to limit the amount of debug information output into a .o file.

    The caching is a bit complicated though so any thoughts on
    untangling that are welcome."

with a fix:

 - Remove all RAUW during type construction by adding stub versions
   of types that we later complete.

and some TODOs:

 - Add an RAUW cache for forward declared types so that we can replace
   them at the end of compilation.
 - Remove the code that updates on completed types because we no
   longer need to have that happen. We emit incomplete types on
   purpose and only want to know when we want to complete them.

llvm-svn: 150752