ast::TypesBuilder: Change const fields to getters

This is required in order to support move operators for TypesBuilder.

Bug: tint:390
Change-Id: I9667bda5f5be267df092f5cd94dc40db053ae6e2
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/38555
Reviewed-by: dan sinclair <dsinclair@chromium.org>

src/scope_stack_test.cc

diff --git a/src/scope_stack_test.cc b/src/scope_stack_test.cc
index 9d45252..9103896 100644
--- a/src/scope_stack_test.cc
+++ b/src/scope_stack_test.cc
@@ -35,7 +35,7 @@
 }
 
 TEST_F(ScopeStackTest, Global_SetWithPointer) {
-  auto* v = Var("my_var", ast::StorageClass::kNone, ty.f32);
+  auto* v = Var("my_var", ast::StorageClass::kNone, ty.f32());
   ScopeStack<ast::Variable*> s;
   s.set_global(v->symbol(), v);