ast: Add address-of and dereference unary ops

Nothing currently generates these.
Resolver currently doesn't know how to handle these.
Backends currently stubbed enough to build without warnings.

Bug: tint:727
Change-Id: I4b9863ae098b903b51a63c36c10bc6e390efbbb3
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/50746
Commit-Queue: Ben Clayton <bclayton@google.com>
Reviewed-by: David Neto <dneto@google.com>
Reviewed-by: James Price <jrprice@google.com>

src/ast/pointer.h

@@ -26,7 +26,7 @@ namespace ast {
 /// A pointer type.
 class Pointer : public Castable<Pointer, Type> {
  public:
-  /// Construtor
+  /// Constructor
   /// @param program_id the identifier of the program that owns this node
   /// @param source the source of this node
   /// @param subtype the pointee type

src/ast/unary_op.cc

@@ -19,6 +19,14 @@ namespace ast {
 
 std::ostream& operator<<(std::ostream& out, UnaryOp mod) {
   switch (mod) {
+    case UnaryOp::kAddressOf: {
+      out << "address-of";
+      break;
+    }
+    case UnaryOp::kDereference: {
+      out << "dereference";
+      break;
+    }
     case UnaryOp::kNegation: {
       out << "negation";
       break;

src/ast/unary_op.h

@@ -21,7 +21,12 @@ namespace tint {
 namespace ast {
 
 /// The unary op
-enum class UnaryOp { kNegation = 0, kNot };
+enum class UnaryOp {
+  kAddressOf,    // &EXPR
+  kDereference,  // *EXPR
+  kNegation,     // -EXPR
+  kNot,          // !EXPR
+};
 
 /// @param out the std::ostream to write to
 /// @param mod the UnaryOp