Handle references to global lets and enclosing locals. (#8061)

When building an expression referencing the result of name lookup, if
the lookup result is non-constant then check we can access it from the
current scope.

* If it is a global, then use a new `GlobalRef` instruction to get the
value of that global. In lowering, create global variables to hold the
values of global `let` bindings so that `GlobalRef` can access those
values.
* If it is a local, walk the lexical scopes between it and the point of
use and ensure it is usable through from all of them. Error if there's
an intervening declarative scope such as a class, add a TODO to handle
captures for intervening functions, and allow `typeof(...)` to name
things it can't read the value of since it's unevaluated.

Global `let`s (which includes `let`s at class scope) now also have their
initializers collected into the global inits block. If the initializer
is non-constant, the wrapper binding also lives in the global inits
block; otherwise it continues to live in the file block and the variable
emitted for it is a global constant.

Assisted-by: Gemini and Claude via Antigravity

toolchain/sem_ir/stringify.cpp

@@ -877,6 +877,10 @@ class Stringifier {
     sem_ir_->ints().Get(inst.int_id).print(*out_, /*isSigned=*/true);
   }
 
+  auto StringifyInst(InstId /*inst_id*/, GlobalRef inst) -> void {
+    step_stack_->PushInstId(inst.inst_id);
+  }
+
   auto StringifyInst(InstId /*inst_id*/, IdentifiedWitness inst) -> void {
     step_stack_->Push(
         inst.query_self_inst_id, " as ",