Make appending var linking less of a special case.

It has to be a bit special because:
* materializeInitFor is not really supposed to call replaceAllUsesWith.
  The caller has a plain variable with Dst and expects just the
  initializer to be set, not for it to be removed.
* Calling mutateType as we used to do before gets some type
  inconsistency which breaks the bitcode writer.
* If linkAppendingVarProto create a dest decl with the correct type to
  avoid the above problems, it needs to put the original dst init in
  some side table for materializeInitFor to use.

In the end the simplest solution seems to be to just have
linkAppendingVarProto do all the work and set ValueMap[SrcGV to avoid
recursion.

llvm-svn: 254424