[clang][CodeGen] Add default constructor for Address. NFC.

[wingo]

Summary

This allows a declaration of an Address to be invalid by default,
allowing it to be added to e.g. a DenseMap. Will followup with a
cleanup.

[wingo]

clang/lib/CodeGen/Address.h

31

It would be nice to assert(isValid()) here but that's not how Address is used; you can pass in a nullptr as the first argument.

[rjmccall]

You can still use a type without a default constructor in a DenseMap; you just have to use insert instead of dict[key] = ....

[wingo]

In D108458#2957789, @rjmccall wrote:

You can still use a type without a default constructor in a DenseMap; you just have to use insert instead of dict[key] = ....

Aaaah, thanks for this note. Closing this PR, then.

clang/lib/CodeGen/Address.h

 #include "llvm/IR/Constants.h"
 #include "clang/AST/CharUnits.h"
 
 namespace clang {
 namespace CodeGen {
 
 /// An aligned address.
 class Address {
-  llvm::Value *Pointer;
+  llvm::Value *Pointer = nullptr;
   CharUnits Alignment;
 public:
+  Address() { assert(!isValid()); }
   Address(llvm::Value *pointer, CharUnits alignment)
       : Pointer(pointer), Alignment(alignment) {
     assert((!alignment.isZero() || pointer == nullptr) &&
            "creating valid address with invalid alignment");
   }
 
-  static Address invalid() { return Address(nullptr, CharUnits()); }
+  static Address invalid() { return Address(); }
   bool isValid() const { return Pointer != nullptr; }
 
   llvm::Value *getPointer() const {
     assert(isValid());
     return Pointer;
   }
 
   /// Return the type of the pointer value.