lazy field index (#4514)

We considered a couple of other options for this:
* https://github.com/carbon-language/carbon-lang/pull/4515 Keep the
`ElementIndex` numbering vptr-ignorant, and do +1 offsets as needed -
seems subtle/easy to miss
* https://github.com/carbon-language/carbon-lang/pull/4517 Always have a
zeroth element in the object representation, make it zero-size in the
case of no-vptr - @zygoloid was concerned this would add overhead
especially to stateless objects used in type-trait-like things.

But currently moving forward with this direction - of initializing field
indexes with an invalid value until the end of the class definition,
then assigning field indexes during construction of the class's object
representation struct type. This direction might reinforce/help avoid
premature access to the object representation before the class is
complete, and give a single place where class layout is done (at class
completion) if we want to add more options there, such as class layout
optimizations, etc.

This patch still has problems with object initialization (that #4515
does not have/does address) but does address normal `obj.member` access
correctly.

---------

Co-authored-by: Richard Smith <richard@metafoo.co.uk>

toolchain/check/handle_function.cpp

@@ -212,8 +212,8 @@ static auto BuildFunctionDecl(Context& context,
       parent_scope_inst) {
     if (auto class_decl = parent_scope_inst->TryAs<SemIR::ClassDecl>()) {
       auto& class_info = context.classes().Get(class_decl->class_id);
-      CARBON_CHECK(virtual_modifier != SemIR::Function::VirtualModifier::Impl ||
-                   class_info.is_dynamic);
+      // TODO: If this is an `impl` function, check there's a matching base
+      // function that's impl or virtual.
       class_info.is_dynamic = true;
     }
   }