Split Program into Program and ProgramBuilder

Program is now immutable*, and remains part of the public Tint
interface.

ProgramBuilder is the mutable builder for Programs, and is not part of
the public Tint interface. ast::Builder has been folded into
ProgramBuilder.

Immutable Programs can be cloned into a mutable ProgramBuilder with
Program::CloneAsBuilder().

Mutable ProgramBuilders can be moved into immutable Programs.

* - mostly immutable. It still has a move constructor and move
  assignment operator - required for practical usage - and the
  semantic information on AST nodes is still mutable.

Bug: tint:390
Change-Id: Ia856c50b1880c2f95c91467a9eef5024cbc380c6
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/38240
Reviewed-by: dan sinclair <dsinclair@chromium.org>

fuzzers/tint_ast_clone_fuzzer.cc

@@ -56,7 +56,7 @@ extern "C" int LLVMFuzzerTestOneInput(const uint8_t* data, size_t size) {
   auto src = parser.program();
 
   // Clone the src program to dst
-  auto dst = src.Clone();
+  tint::Program dst(src.Clone());
 
   // Expect the demangled AST printed with to_str() to match
   tint::Demangler d;

fuzzers/tint_common_fuzzer.cc

@@ -19,6 +19,10 @@
 #include <utility>
 #include <vector>
 
+#include "src/ast/module.h"
+#include "src/program.h"
+#include "src/program_builder.h"
+
 namespace tint {
 namespace fuzzers {
 
@@ -81,9 +85,13 @@ int CommonFuzzer::Run(const uint8_t* data, size_t size) {
     return 0;
   }
 
-  TypeDeterminer td(&program);
-  if (!td.Determine()) {
-    return 0;
+  {
+    ProgramBuilder builder = program.CloneAsBuilder();
+    TypeDeterminer td(&builder);
+    if (!td.Determine()) {
+      return 0;
+    }
+    program = Program(std::move(builder));
   }
 
   Validator v;