Fix: Resolve compilation failure in F14Table.h due to two-phase lookup

Summary:
Hi, this PR fixes a compilation issue in `folly/container/detail/F14Table.h`.

### Bug Description

When building Folly with a modern g++ toolchain, the compilation fails with multiple errors related to `occupiedMask`.

**Environment:**

  * **OS:** Ubuntu 24.04
  * **g++ version:** 13.3.0
  * **CPU:** Intel(R) Xeon(R) E-2286M CPU @ 2.40GHz

**Key Error Message:**

```
/root/folly/folly/container/detail/F14Table.h:821:37: error: there are no arguments to 'occupiedMask' that depend on a template parameter, so a declaration of 'occupiedMask' must be available [-fpermissive]
   821 |     return DenseMaskIter{&tags_[0], occupiedMask()};
       |                                     ^~~~~~~~~~~~
```

### Root Cause

The error is a classic C++ two-phase name lookup problem. Inside the `F14Chunk<ItemType>` template, functions like `occupiedIter()` call `occupiedMask()` without any qualification. Because `occupiedMask` is a member function of `F14Chunk`, its existence depends on the template parameter `ItemType`. According to the rules, the compiler needs to be explicitly told that `occupiedMask` is a dependent name, which g++ 13.3.0 correctly enforces.

### The Fix

The solution is to explicitly qualify the calls to `occupiedMask` with `this->`. This informs the compiler that `occupiedMask` is a member of the template instance and its lookup should be deferred to the instantiation phase.

This change is applied to all calls to `occupiedMask` within `F14Chunk` in `folly/container/detail/F14Table.h`.

### How to Test

The fix can be verified by successfully compiling the `folly_base` target in the described environment (Ubuntu 24.04, g++ 13.3.0) where the build was previously failing. All existing tests should continue to pass.

Thanks for your review\!During compilation with certain toolchains (e.g., g++), the build fails with an "'occupiedMask' was not declared in this scope" error inside the F14Chunk template class.

This is caused by C++'s two-phase name lookup rules for templates, where non-dependent names are resolved during the initial template parsing. The compiler fails to find 'occupiedMask' as it is a member of a dependent base class.

This commit resolves the build failure by explicitly qualifying the member function calls with 'this->', correctly hinting to the compiler that 'occupiedMask' is a dependent name that should be looked up during template instantiation.

X-link: facebook/folly#2491

Reviewed By: dmm-fb

Differential Revision: D93942306

Pulled By: yfeldblum

fbshipit-source-id: 3a93ac95efd90ed733041eff0f051a620db7f025

Author: ihb2032

third-party/folly/src/folly/container/detail/F14Table.h

@@ -814,19 +814,19 @@ struct alignas(kRequiredVectorAlignment) F14Chunk {
 #endif
 
   DenseMaskIter occupiedIter() const {
-    return DenseMaskIter{&tags_[0], occupiedMask()};
+    return DenseMaskIter{&tags_[0], this->occupiedMask()};
   }
 
   MaskRangeIter occupiedRangeIter() const {
-    return MaskRangeIter{occupiedMask()};
+    return MaskRangeIter{this->occupiedMask()};
   }
 
   LastOccupiedInMask lastOccupied() const {
-    return LastOccupiedInMask{occupiedMask()};
+    return LastOccupiedInMask{this->occupiedMask()};
   }
 
   FirstEmptyInMask firstEmpty() const {
-    return FirstEmptyInMask{occupiedMask() ^ kFullMask};
+    return FirstEmptyInMask{this->occupiedMask() ^ kFullMask};
   }
 
   bool occupied(std::size_t index) const {