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/SILFunction.cpp

@@ -98,7 +98,7 @@ SILFunction::SILFunction(SILModule &Module, SILLinkage Linkage, StringRef Name,
       Serialized(isSerialized), Thunk(isThunk),
       ClassSubclassScope(unsigned(classSubclassScope)), GlobalInitFlag(false),
       InlineStrategy(inlineStrategy), Linkage(unsigned(Linkage)),
-      HasCReferences(false),
+      HasCReferences(false), IsWeakLinked(false),
       OptMode(OptimizationMode::NotSet), EffectsKindAttr(E),
       EntryCount(entryCount) {
   if (InsertBefore)