Remove ast::AccessControl::kInvalid

Also spruce up texture validation tests so that they validate error
messages.

Bug: tint:805
Change-Id: I6c86fc16014b127a7ef8254e5badf9b5bed08623
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/51860
Reviewed-by: Ben Clayton <bclayton@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Antonio Maiorano <amaiorano@google.com>

src/ast/access_control.cc

@@ -37,9 +37,6 @@ AccessControl::~AccessControl() = default;
 std::string AccessControl::type_name() const {
   std::string name = "__access_control_";
   switch (access_) {
-    case ast::AccessControl::kInvalid:
-      name += "invalid";
-      break;
     case ast::AccessControl::kReadOnly:
       name += "read_only";
       break;
@@ -57,9 +54,6 @@ std::string AccessControl::FriendlyName(const SymbolTable& symbols) const {
   std::ostringstream out;
   out << "[[access(";
   switch (access_) {
-    case ast::AccessControl::kInvalid:
-      out << "invalid";
-      break;
     case ast::AccessControl::kReadOnly:
       out << "read";
       break;
@@ -83,10 +77,6 @@ AccessControl* AccessControl::Clone(CloneContext* ctx) const {
 
 std::ostream& operator<<(std::ostream& out, AccessControl::Access access) {
   switch (access) {
-    case ast::AccessControl::kInvalid: {
-      out << "invalid";
-      break;
-    }
     case ast::AccessControl::kReadOnly: {
       out << "read_only";
       break;

src/ast/access_control.h

@@ -28,9 +28,6 @@ class AccessControl : public Castable<AccessControl, Type> {
  public:
   /// The access control settings
   enum Access {
-    /// Invalid
-    // TODO(crbug.com/tint/805): Remove this.
-    kInvalid = -1,
     /// Read only
     kReadOnly,
     /// Write only