IR: Call dropAllReferences from GlobalVariable's destructor.

[pcc]

Summary

We were previously failing to do this and as a result failing to drop
attached metadata.

Not sure if there's a good way to test this. An in-progress patch exposed this
problem by allocating a GlobalVariable at the same address as a previously
allocated GlobalVariable.

[aprantl]

Seems reasonable...
Is that only a problem in GlobalVariable or also in its base classes?

[pcc]

It's only a problem for classes that need to override dropAllReferences. Most classes are fine with the User::~User behaviour, which just clears operands using User::dropAllReferences.

include/llvm/IR/GlobalVariable.h

public:
   /// specified other global.
   GlobalVariable(Module &M, Type *Ty, bool isConstant,
                  LinkageTypes Linkage, Constant *Initializer,
                  const Twine &Name = "", GlobalVariable *InsertBefore = nullptr,
                  ThreadLocalMode = NotThreadLocal, unsigned AddressSpace = 0,
                  bool isExternallyInitialized = false);
 
   ~GlobalVariable() override {
+    dropAllReferences();
+
     // FIXME: needed by operator delete
     setGlobalVariableNumOperands(1);
   }
 
   /// Provide fast operand accessors
   DECLARE_TRANSPARENT_OPERAND_ACCESSORS(Value);
 
   /// Definitions have initializers, declarations don't.