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/check/full_pattern_stack.cpp

@@ -10,8 +10,7 @@
 namespace Carbon::Check {
 
 auto FullPatternStack::StartPatternInitializer() -> void {
-  CARBON_CHECK(kind_stack_.back() == Kind::ClassScopeVarDecl ||
-               kind_stack_.back() == Kind::NameBindingDecl);
+  CARBON_CHECK(IsAnyNameBindingDeclKind(kind_stack_.back()));
   for (auto& [name_id, inst_id] : bind_name_stack_.PeekArray()) {
     CARBON_CHECK(
         inst_id == SemIR::InstId::InitTombstone,