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/AST/DeclContext.cpp

@@ -478,7 +478,7 @@ swift::FragileFunctionKindRequest::evaluate(Evaluator &evaluator,
                 /*allowUsableFromInline=*/true};
       }
 
-      if (AFD->getAttrs().hasAttribute<BackDeployedAttr>()) {
+      if (AFD->isBackDeployed(context->getASTContext())) {
         return {FragileFunctionKind::BackDeploy,
                 /*allowUsableFromInline=*/true};
       }
@@ -495,7 +495,7 @@ swift::FragileFunctionKindRequest::evaluate(Evaluator &evaluator,
           return {FragileFunctionKind::AlwaysEmitIntoClient,
                   /*allowUsableFromInline=*/true};
         }
-        if (storage->getAttrs().hasAttribute<BackDeployedAttr>()) {
+        if (storage->isBackDeployed(context->getASTContext())) {
           return {FragileFunctionKind::BackDeploy,
                   /*allowUsableFromInline=*/true};
         }