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/inspector/inspector.cc

@@ -45,6 +45,8 @@ namespace inspector {
 
 Inspector::Inspector(const ast::Module& module) : module_(module) {}
 
+Inspector::Inspector(const Program* program) : Inspector(program->module) {}
+
 Inspector::~Inspector() = default;
 
 std::vector<EntryPoint> Inspector::GetEntryPoints() {