[modules] Defer setting up the lookup table for a DeclContext until we can

determine the primary context, rather than sometimes registering the lookup
table on the wrong context.

This exposed a couple of bugs:
 * the odr violation check didn't deal properly with mergeable declarations
   if the declaration retained by name lookup wasn't in the canonical
   definition of the class
 * the (broken) RewriteDecl mechanism would emit two name lookup tables for
   the same DeclContext into the same module file (one as part of the
   rewritten declaration and one as a visible update for the old declaration)
These are both fixed too.

llvm-svn: 244192