Introduce ObjCRuntime::hasWeakClassImport() and use it in the appropriate
places.  I've turned this off for the GNU runtimes --- I don't know if
they support weak class import, but it's easy enough for them to opt in.

Also tweak a comment per review by Jordan.

llvm-svn: 158860
diff --git a/clang/lib/AST/DeclBase.cpp b/clang/lib/AST/DeclBase.cpp
index 7d72093..574f779 100644
--- a/clang/lib/AST/DeclBase.cpp
+++ b/clang/lib/AST/DeclBase.cpp
@@ -430,7 +430,7 @@
 
   // Objective-C classes, if this is the non-fragile runtime.
   } else if (isa<ObjCInterfaceDecl>(this) &&
-             getASTContext().getLangOpts().ObjCRuntime.isNonFragile()) {
+             getASTContext().getLangOpts().ObjCRuntime.hasWeakClassImport()) {
     return true;
 
   // Nothing else.