Support for initialization of classes with abstract base classes. (#6802)

When initializing `.base` in class initialization, use `partial Base` as
the destination type rather than `Base`. Treat `partial Base` as not
being abstract even when `Base` is.

Allow conversion from a `partial T` initializer to a `T` initializer.
Store the vptr while performing the conversion. Do not store the vptr
when performing a `partial T` initialization, only when performing a
non-partial `T` initialization.

toolchain/check/eval_inst.cpp

@@ -773,6 +773,13 @@ auto EvalConstantInst(Context& context, SemIR::UnaryOperatorNot inst)
   return ConstantEvalResult::NotConstant;
 }
 
+auto EvalConstantInst(Context& /*context*/, SemIR::UpdateInit /*inst*/)
+    -> ConstantEvalResult {
+  // TODO: Support folding together a ClassInit with an update that sets the
+  // vptr.
+  return ConstantEvalResult::TODO;
+}
+
 auto EvalConstantInst(Context& context, SemIR::ValueOfInitializer inst)
     -> ConstantEvalResult {
   // Values of value expressions and initializing expressions are represented in