[modules] Fix some visibility issues with default template arguments.

There are still problems here, but this is a better starting point.

The main part of the change is: when doing a lookup that would accept visible
or hidden declarations, prefer to produce the latest visible declaration if
there are any visible declarations, rather than always producing the latest
declaration.

Thus, when we inherit default arguments (and other properties) from a previous
declaration, we inherit them from the previous visible declaration; if the
previous declaration is hidden, we already suppress inheritance of default
arguments.

There are a couple of other changes here that fix latent bugs exposed by this
change.

llvm-svn: 239371