Consistently create a new declaration when merging a pre-existing but

Consistently create a new declaration when merging a pre-existing but
hidden definition with a would-be-parsed redefinition.

This permits a bunch of cleanups. In particular, we no longer need to
take merged definitions into account when checking declaration
visibility, only when checking definition visibility, which makes
certain visibility checks take linear instead of quadratic time.

We could also now remove the UPD_DECL_EXPORTED update record and track
on each declaration whether it was demoted from a definition (as we
already do for variables), but I'm not doing that in this patch to keep
the changes here simpler.

llvm-svn: 342018