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>
diff --git a/src/reader/wgsl/parser_impl_inclusive_or_expression_test.cc b/src/reader/wgsl/parser_impl_inclusive_or_expression_test.cc
index 9a23a38..6150645 100644
--- a/src/reader/wgsl/parser_impl_inclusive_or_expression_test.cc
+++ b/src/reader/wgsl/parser_impl_inclusive_or_expression_test.cc
@@ -39,7 +39,7 @@
 
   ASSERT_TRUE(rel->lhs()->Is<ast::IdentifierExpression>());
   auto* ident = rel->lhs()->As<ast::IdentifierExpression>();
-  EXPECT_EQ(ident->symbol(), p->get_program().Symbols().Register("a"));
+  EXPECT_EQ(ident->symbol(), p->builder().Symbols().Get("a"));
 
   ASSERT_TRUE(rel->rhs()->Is<ast::ConstructorExpression>());
   ASSERT_TRUE(rel->rhs()->Is<ast::ScalarConstructorExpression>());