Start adding builtins to SemanticsIR (#2356)

This starts adding builtins with TypeType and IntegerLiteralType. Note, structurally that's all they are, and not directly accessible in any way.

Adds a type field to SemanticsNode. Now, IntegerLiteralType can be identified as having type=TypeType, and IntegerLiteral as type=IntegerLiteralType. The current iteration doesn't do anything for type propagation, because I wanted to avoid making this too big.

This also switches the Identifier IR to instead BindName, with some side-effects. I'd been trying to think how to provide a name for TypeType, and switching around how things worked seemed like a better approach. And while I think it's the right direction (e.g., alias should just be a BindName), I also realized I don't need to name TypeType: there's probably a keyword to refer to the builtin, so it shouldn't use regular name lookup.

toolchain/semantics/semantics_node.cpp

@@ -4,6 +4,8 @@
 
 #include "toolchain/semantics/semantics_node.h"
 
+#include "toolchain/semantics/semantics_builtin_kind.h"
+
 namespace Carbon {
 
 static auto PrintArgs(llvm::raw_ostream& /*out*/,
@@ -29,6 +31,9 @@ void SemanticsNode::Print(llvm::raw_ostream& out) const {
 #include "toolchain/semantics/semantics_node_kind.def"
   }
   out << ")";
+  if (type_.id != -1) {
+    out << ": " << type_;
+  }
 }
 
 }  // namespace Carbon