Remove unused GlobalVariable::replaceUsesOfWithOnConstant. NFC.

The only caller of this method is Value::replaceAllUsesWith which
explicitly checks that we are not a GlobalValue.  So replace the
body with an unreachable to ensure that we never call it.

The unreachable itself is moved to GlobalValue not GlobalVariable
as that is the base class of all the globals we don't want to call
this method on.

Note, this patch is short lived as i'll soon refactor all callers
of this method.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@240486 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/IR/GlobalVariable.h

@@ -166,10 +166,6 @@ public:
   ///
   void eraseFromParent() override;
 
-  /// Override Constant's implementation of this method so we can
-  /// replace constant initializers.
-  void replaceUsesOfWithOnConstant(Value *From, Value *To, Use *U) override;
-
   // Methods for support type inquiry through isa, cast, and dyn_cast:
   static inline bool classof(const Value *V) {
     return V->getValueID() == Value::GlobalVariableVal;