Superficial support for Core.BigInt type (#4414)

Add a `Core.BigInt` type and a corresponding builtin type in the
toolchain. See [corresponding section of the
design](https://docs.carbon-lang.dev/docs/design/expressions/literals.html#defined-types).

So far this type is not used for anything, and there is no way to create
an instance of it.

toolchain/sem_ir/builtin_function_kind.cpp

@@ -173,6 +173,10 @@ constexpr BuiltinInfo None = {"", nullptr};
 constexpr BuiltinInfo PrintInt = {"print.int",
                                   ValidateSignature<auto(AnyInt)->NoReturn>};
 
+// Returns the `Core.BigInt` type.
+constexpr BuiltinInfo BigIntMakeType = {"big_int.make_type",
+                                        ValidateSignature<auto()->Type>};
+
 // Returns the `i32` type. Doesn't take a bit size because we need an integer
 // type as a basis for that.
 constexpr BuiltinInfo IntMakeType32 = {"int.make_type_32",