Make AsyncStackRoot::getTopFrame() const

Summary:
This is needed if we have an AsyncStackRoot pointer, and we
want to call both `getTopFrame()` and `getNextRoot()`
(e.g. walking async stacks) without const-casting the pointer.

Reviewed By: yfeldblum

Differential Revision: D26675131

fbshipit-source-id: 84de25c67d563e118483d67d0fedb4ea7aef2604

folly/tracing/AsyncStack-inl.h

@@ -108,7 +108,7 @@ inline void AsyncStackRoot::setTopFrame(AsyncStackFrame& frame) noexcept {
   this->topFrame.store(&frame, std::memory_order_release);
 }
 
-inline AsyncStackFrame* AsyncStackRoot::getTopFrame() noexcept {
+inline AsyncStackFrame* AsyncStackRoot::getTopFrame() const noexcept {
   return topFrame.load(std::memory_order_relaxed);
 }
 

folly/tracing/AsyncStack.h

@@ -377,7 +377,7 @@ struct AsyncStackRoot {
   // The current stack root must not currently have any active
   // frame.
   void setTopFrame(AsyncStackFrame& frame) noexcept;
-  AsyncStackFrame* getTopFrame() noexcept;
+  AsyncStackFrame* getTopFrame() const noexcept;
 
   // Initialises this stack root with information about the context
   // in which the stack-root was declared. This records information