Add @_weakLinked and a corresponding SIL attribute

This is mostly intended to be used for testing at this point; in the
long run, we want to be using availability information to decide
whether to weak-link something or not. You'll notice a bunch of FIXMEs
in the test case that we may not need now, but will probably need to
handle in the future.

Groundwork for doing backward-deployment execution tests.

lib/SIL/SILModule.cpp

@@ -310,6 +310,9 @@ static void addFunctionAttributes(SILFunction *F, DeclAttributes &Attrs,
   if (Attrs.hasAttribute<SILGenNameAttr>() ||
       Attrs.hasAttribute<CDeclAttr>())
     F->setHasCReferences(true);
+
+  if (Attrs.hasAttribute<WeakLinkedAttr>())
+    F->setWeakLinked();
 }
 
 SILFunction *SILModule::getOrCreateFunction(SILLocation loc,