AST: Only treat @backDeployed functions as fragile on platforms with an active attribute.

Previously, typechecking and SILGen would treat a function body as fragile as long as the declaration had a `@backDeployed` attribute, regardless of the platform specified by the attribute. This was overly conservative since back deployed functions are only emitted into the client on specific platforms. Now a `@backDeployed` function can reference non-`public` declarations on the platforms it is resilient on:

```
@backDeployed(before: iOS 15)
public func foo() {
  #if os(iOS)
  // Fragile; this code may be emitted into the client.
  #else
  // Resilient; this code won't ever be exposed to clients.
  #endif
}
```

Resolves rdar://105298520

lib/SIL/IR/SILDeclRef.cpp

@@ -955,7 +955,7 @@ bool SILDeclRef::isBackDeployed() const {
 
   auto *decl = getDecl();
   if (auto afd = dyn_cast<AbstractFunctionDecl>(decl))
-    return afd->isBackDeployed();
+    return afd->isBackDeployed(getASTContext());
 
   return false;
 }