SIL: Don't descend into nominal types' parents to determine linkage.

A type in an extension doesn't inherit linkage from its parent. In particular, if an extension adds a nested type to an imported class, we don't want that nested type to have unique linkage. Fixes rdar://problem/19792174.

Swift SVN r25191

lib/SIL/SIL.cpp

@@ -100,6 +100,9 @@ FormalLinkage swift::getTypeLinkage(CanType type) {
     // For any nominal type reference, look at the type declaration.
     if (auto nominal = type->getAnyNominal()) {
       result ^= getDeclLinkage(nominal);
+      // The generic parameters or parent of the nominal type don't affect the
+      // declaration's linkage. We can stop.
+      return true;
 
     // For polymorphic function types, look at the generic parameters.
     // FIXME: findIf should do this, once polymorphic function types can be