Add an interface for initialization of vars without an explicit initializer (#6934)

When a `var` is not explicitly given an initializer, initialize it in
one of two ways:

* If its type implements the new interface `Core.Default`, call
`Core.Default.Op` to initialize it.
* Otherwise, if its type implements `UnformedInit`, leave it in an
unformed state. For now, this is always an uninitialized state, but that
will change in the future.
* If neither of those apply, the `var` declaration is ill-formed.

This is a step towards implementing leads decision #6739 and proposals
#257 and #5913.

Assisted-by: Gemini 3.1 Pro via Antigravity

---------

Co-authored-by: Geoff Romer <gromer@google.com>

toolchain/sem_ir/builtin_function_kind.cpp

@@ -400,6 +400,9 @@ constexpr BuiltinInfo None = {"", nullptr};
 
 constexpr BuiltinInfo NoOp = {"no_op", ValidateNoOpSignature};
 
+constexpr BuiltinInfo MakeUninitialized = {"make_uninitialized",
+                                           ValidateSignature<auto()->AnyType>};
+
 constexpr BuiltinInfo PrimitiveCopy = {
     "primitive_copy",
     ValidateSignature<auto(PrimitiveCopyParamT)->PrimitiveCopyParamT>};