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>
diff --git a/src/ast/null_literal_test.cc b/src/ast/null_literal_test.cc
index 93b6d44..e50bd42 100644
--- a/src/ast/null_literal_test.cc
+++ b/src/ast/null_literal_test.cc
@@ -28,7 +28,7 @@
 using NullLiteralTest = TestHelper;
 
 TEST_F(NullLiteralTest, Is) {
-  ast::Literal* l = create<NullLiteral>(ty.i32);
+  ast::Literal* l = create<NullLiteral>(ty.i32());
   EXPECT_FALSE(l->Is<BoolLiteral>());
   EXPECT_FALSE(l->Is<SintLiteral>());
   EXPECT_FALSE(l->Is<FloatLiteral>());
@@ -38,13 +38,13 @@
 }
 
 TEST_F(NullLiteralTest, ToStr) {
-  auto* i = create<NullLiteral>(ty.i32);
+  auto* i = create<NullLiteral>(ty.i32());
 
   EXPECT_EQ(i->to_str(), "null __i32");
 }
 
 TEST_F(NullLiteralTest, Name_I32) {
-  auto* i = create<NullLiteral>(ty.i32);
+  auto* i = create<NullLiteral>(ty.i32());
   EXPECT_EQ("__null__i32", i->name());
 }