Annotate net-new Snapshot-borrow accessors with [[clang::lifetimebound]]

Summary:
Adds [[clang::lifetimebound]] to the borrow-returning folly::observer snapshot accessors NOT
already covered by the D109704537 stack, so the compiler (-Wdangling / -Wreturn-stack-address)
catches consumers that bind a reference/pointer into a snapshot that does not outlive it:

  - TLObserver::getSnapshotRef() / operator*()              (Observer.h)
  - HazptrObserver::HazptrSnapshot operator*/->/get         (HazptrObserver.h)
  - CoreCachedObserver::CoreCachedSnapshot operator*/->/get (CoreCachedObserver.h)

Snapshot::operator*/->/get are annotated separately by the D109704537 stack.

The single consumer this surfaced -- luna/quorumlib QuorumConfig::getConfigName returning a
const std::string& into a temporary snapshot -- is fixed in the parent diff D109961109; with it
in the stack, full CI is green.

Reviewed By: r-barnes

Differential Revision: D109953065

fbshipit-source-id: 8e9cb2d93b658960e0a7d1f94867910664be9741

Author: rikvanriel

folly/observer/CoreCachedObserver.h

@@ -18,6 +18,7 @@
 
 #include <memory>
 
+#include <folly/CppAttributes.h>
 #include <folly/concurrency/CoreCachedSharedPtr.h>
 #include <folly/observer/Observer.h>
 #include <folly/observer/detail/ObserverManager.h>
@@ -31,9 +32,15 @@ class CoreCachedObserver {
     explicit CoreCachedSnapshot(std::shared_ptr<const T> data)
         : data_(std::move(data)) {}
 
-    const T& operator*() const { return *get(); }
-    const T* operator->() const { return get(); }
-    const T* get() const { return data_.get(); }
+    const T& operator*() const [[FOLLY_ATTR_CLANG_LIFETIMEBOUND]] {
+      return *get();
+    }
+    const T* operator->() const [[FOLLY_ATTR_CLANG_LIFETIMEBOUND]] {
+      return get();
+    }
+    const T* get() const [[FOLLY_ATTR_CLANG_LIFETIMEBOUND]] {
+      return data_.get();
+    }
 
     std::shared_ptr<const T> getShared() const& { return data_; }
     std::shared_ptr<const T> getShared() && { return std::move(data_); }

folly/observer/HazptrObserver.h

@@ -19,6 +19,7 @@
 #include <atomic>
 #include <memory>
 
+#include <folly/CppAttributes.h>
 #include <folly/Synchronized.h>
 #include <folly/observer/Observer.h>
 #include <folly/observer/detail/ObserverManager.h>
@@ -57,9 +58,13 @@ class HazptrObserver {
       ptr_ = get(holder_).protect(state)->snapshot_.get();
     }
 
-    const T& operator*() const { return *get(); }
-    const T* operator->() const { return get(); }
-    const T* get() const { return ptr_; }
+    const T& operator*() const [[FOLLY_ATTR_CLANG_LIFETIMEBOUND]] {
+      return *get();
+    }
+    const T* operator->() const [[FOLLY_ATTR_CLANG_LIFETIMEBOUND]] {
+      return get();
+    }
+    const T* get() const [[FOLLY_ATTR_CLANG_LIFETIMEBOUND]] { return ptr_; }
 
    private:
     static void make(hazptr_holder<Atom>& holder, hazptr_domain<Atom>& domain) {

folly/observer/Observer.h

@@ -19,6 +19,7 @@
 #include <atomic>
 #include <memory>
 
+#include <folly/CppAttributes.h>
 #include <folly/SharedMutex.h>
 #include <folly/ThreadLocal.h>
 #include <folly/observer/Observer-pre.h>
@@ -418,8 +419,10 @@ class TLObserver {
   TLObserver(const TLObserver<T>& other);
   TLObserver(TLObserver<T>&& other) noexcept;
 
-  const Snapshot<T>& getSnapshotRef() const;
-  const Snapshot<T>& operator*() const { return getSnapshotRef(); }
+  const Snapshot<T>& getSnapshotRef() const [[FOLLY_ATTR_CLANG_LIFETIMEBOUND]];
+  const Snapshot<T>& operator*() const [[FOLLY_ATTR_CLANG_LIFETIMEBOUND]] {
+    return getSnapshotRef();
+  }
 
   /**
    * Invoke a function with the current observed value. The snapshot is held