[Modules] Fix overly conservative assertion for import diagnostic

We currenltly assert when want to diagnose a missing import and the decl
in question is already visible. It turns out that the decl in question
might be visible because another decl from the same module actually made
the module visible in a previous error diagnostic.

Remove the assertion and avoid re-exporting the module if it's already
visible.

rdar://problem/27975402

Differential Revision: https://reviews.llvm.org/D32828

llvm-svn: 303705