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/reader/reader.h

@@ -21,6 +21,7 @@
 #include "src/ast/module.h"
 #include "src/diagnostic/diagnostic.h"
 #include "src/diagnostic/formatter.h"
+#include "src/program.h"
 
 namespace tint {
 namespace reader {
@@ -49,6 +50,9 @@ class Reader {
   /// @returns the module. The module in the parser will be reset after this.
   virtual ast::Module module() = 0;
 
+  /// @returns the program. The module in the parser will be reset after this.
+  Program program() { return Program{module()}; }
+
  protected:
   /// Constructor
   Reader();