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/lower/function_context.cpp

@@ -410,18 +410,23 @@ auto FunctionContext::CopyValue(TypeInFile type, SemIR::InstId source_id,
   }
 }
 
-auto FunctionContext::CopyObject(TypeInFile type, SemIR::InstId source_id,
-                                 SemIR::InstId dest_id) -> void {
+auto FunctionContext::CopyObject(TypeInFile type, llvm::Value* source,
+                                 llvm::Value* dest) -> void {
   const auto& layout = llvm_module().getDataLayout();
   auto* llvm_type = GetType(type);
   auto align = GetAlignment(type);
 
   // TODO: Attach !tbaa.struct metadata indicating which portions of the
   // type we actually need to copy and which are padding.
-  builder().CreateMemCpy(GetValue(dest_id), align, GetValue(source_id), align,
+  builder().CreateMemCpy(dest, align, source, align,
                          layout.getTypeAllocSize(llvm_type));
 }
 
+auto FunctionContext::CopyObject(TypeInFile type, SemIR::InstId source_id,
+                                 SemIR::InstId dest_id) -> void {
+  CopyObject(type, GetValue(source_id), GetValue(dest_id));
+}
+
 auto FunctionContext::Inserter::InsertHelper(
     llvm::Instruction* inst, const llvm::Twine& name,
     llvm::BasicBlock::iterator insert_pt) const -> void {