[modules] Add an assert for redeclarations that we never added to their redecl

chain and fix the cases where it fires.

 * Handle the __va_list_tag as a predefined decl. Previously we failed to merge
   sometimes it because it's not visible to name lookup. (In passing, remove
   redundant __va_list_tag typedefs that we were creating for some ABIs. These
   didn't affect the mangling or representation of the type.)

 * For Decls derived from Redeclarable that are not in fact redeclarable
   (implicit params, function params, ObjC type parameters), remove them from
   the list of expected redeclarable decls.

llvm-svn: 243259