[NFC] Cleanup: Function::getBasicBlockList() and Function::getSublistAccess() are now private

[vporpo]

Summary

The Function class now supports an adequate set of member functions to help
modify the BB list, including Function::splice(), Function::erase(),
Function::insert() etc. so we no longer need access to the underlying list.

[vporpo]

Rebase

llvm/include/llvm/IR/Function.h

public:
   void splice(Function::iterator ToIt, Function *FromF,
               Function::iterator FromBeginIt,
               Function::iterator FromEndIt);
 
   /// Erases a range of BasicBlocks from \p FromIt to (not including) \p ToIt.
   /// \Returns \p ToIt.
   Function::iterator erase(Function::iterator FromIt, Function::iterator ToIt);
 
+private:
+  // These need access to the underlying BB list.
+  friend void BasicBlock::removeFromParent();
+  friend iplist<BasicBlock>::iterator BasicBlock::eraseFromParent();
+  template <class BB_t, class BB_i_t, class BI_t, class II_t>
+  friend class InstIterator;
+  friend class llvm::SymbolTableListTraits<llvm::BasicBlock>;
+  friend class llvm::ilist_node_with_parent<llvm::BasicBlock, llvm::Function>;
+
   /// Get the underlying elements of the Function... the basic block list is
   /// empty for external functions.
   ///
   /// This is deliberately private because we have implemented an adequate set
   /// of functions to modify the list, including Function::splice(),
   /// Function::erase(), Function::insert() etc.
   const BasicBlockListType &getBasicBlockList() const { return BasicBlocks; }
         BasicBlockListType &getBasicBlockList()       { return BasicBlocks; }
 
   static BasicBlockListType Function::*getSublistAccess(BasicBlock*) {
     return &Function::BasicBlocks;
   }
 
+public:
   const BasicBlock       &getEntryBlock() const   { return front(); }
         BasicBlock       &getEntryBlock()         { return front(); }
 
   //===--------------------------------------------------------------------===//
   // Symbol Table Accessing functions...
 
   /// getSymbolTable() - Return the symbol table if any, otherwise nullptr.
   ///