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>

src/validator/validator_impl.h

@@ -24,6 +24,7 @@
 #include "src/ast/call_expression.h"
 #include "src/ast/expression.h"
 #include "src/ast/identifier_expression.h"
+#include "src/ast/module.h"
 #include "src/ast/return_statement.h"
 #include "src/ast/statement.h"
 #include "src/ast/switch_statement.h"
@@ -48,6 +49,9 @@ class ValidatorImpl {
   /// @returns true if the validation was successful
   bool Validate();
 
+  /// @returns the program being validated
+  const Program* program() { return program_; }
+
   /// @returns the diagnostic messages
   const diag::List& diagnostics() const { return diags_; }
   /// @returns the diagnostic messages