Add tint::Program as a wrapper of tint::ast::Module.

`tint::Program` will become the new public API object for a parsed shader program.
For now, have Program be a simple wrapper around ast::Module so we can migrate Dawn's use of the public tint API.

Add new Program variants of public APIs for places that returned or took a Module.

Remove Reset() methods from Generators, they aren't used, and make the migration harder.

Change-Id: Ic5bee46ceb109ea591ba7fec33685220b244a1ae
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/38540
Reviewed-by: dan sinclair <dsinclair@chromium.org>
Commit-Queue: Ben Clayton <bclayton@google.com>

src/validator/validator.h

@@ -24,6 +24,7 @@
 #include "src/ast/statement.h"
 #include "src/diagnostic/diagnostic.h"
 #include "src/diagnostic/formatter.h"
+#include "src/program.h"
 
 namespace tint {
 
@@ -39,6 +40,11 @@ class Validator {
   /// @returns true if the validation was successful
   bool Validate(const ast::Module* module);
 
+  /// Runs the validator
+  /// @param program the program to validate
+  /// @returns true if the validation was successful
+  bool Validate(const Program* program) { return Validate(&program->module); }
+
   /// @returns error messages from the validator
   std::string error() {
     diag::Formatter formatter{{false, false, false, false}};