ast: Replace NamedType with TypeDecl

TypeDecls (alias, structure) are not a types - they declare types.
ast::TypeName is what's used for a ast::Type.

Previously we were trying to automatically convert these to TypeNames in the builder, but having these inherit from ast::Type was extremely error prone.

reader/spirv was actually constructing ast::Structs and using them as types, which is invalid.

Change-Id: I05773ad6d488626606019015b84217a5a55a8e8a
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/53802
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>

src/sem/struct.cc

@@ -13,11 +13,13 @@
 // limitations under the License.
 
 #include "src/sem/struct.h"
-#include "src/ast/struct_member.h"
 
 #include <string>
 #include <utility>
 
+#include "src/ast/struct_member.h"
+#include "src/symbol_table.h"
+
 TINT_INSTANTIATE_TYPEINFO(tint::sem::Struct);
 TINT_INSTANTIATE_TYPEINFO(tint::sem::StructMember);
 
@@ -51,7 +53,7 @@ std::string Struct::type_name() const {
 }
 
 std::string Struct::FriendlyName(const SymbolTable& symbols) const {
-  return declaration_->FriendlyName(symbols);
+  return symbols.NameFor(declaration_->name());
 }
 
 StructMember::StructMember(ast::StructMember* declaration,

src/sem/type_mappings.h

@@ -28,6 +28,7 @@ class MemberAccessorExpression;
 class Statement;
 class StructMember;
 class Type;
+class TypeDecl;
 class Variable;
 }  // namespace ast
 
@@ -57,6 +58,7 @@ struct TypeMappings {
   Statement* operator()(ast::Statement*);
   StructMember* operator()(ast::StructMember*);
   Type* operator()(ast::Type*);
+  Type* operator()(ast::TypeDecl*);
   Variable* operator()(ast::Variable*);
   //! @endcond
 };