[modules] When performing a lookup into a namespace, ensure that any later

redefinitions of that namespace have already been loaded. When writing out the
names in a namespace, if we see a name that is locally declared and had
imported declarations merged on top of it, export the local declaration as the
lookup result, because it will be the most recent declaration of that entity in
the redeclaration chain of an importer of the module.

llvm-svn: 215518