Start refactoring type handling to track which types are in-use. (#2848)

This is the first step to refactoring types into a SemanticsTypeId. This only tracks what's in-use, but as a consequence starts funneling type information through in ways similar to how I'd want it to do SemanticsTypeId.

toolchain/semantics/semantics_ir_test.cpp

@@ -54,6 +54,7 @@ TEST(SemanticsIRTest, YAML) {
           Pair("integer_literals", Yaml::Sequence(ElementsAre("0"))),
           Pair("real_literals", Yaml::Sequence(IsEmpty())),
           Pair("strings", Yaml::Sequence(ElementsAre("x"))),
+          Pair("types", Yaml::Sequence(ElementsAre(node_builtin))),
           Pair(
               "nodes",
               Yaml::Sequence(AllOf(