[ty] Implement global handling and load-before-global-declaration syntax error (#17637)

Summary
--

This PR resolves both the typing-related and syntax error TODOs added in
#17563 by tracking a set of `global` bindings for each scope. As
discussed below, we avoid the additional AST traversal from ruff by
collecting `Name`s from `global` statements while building the semantic
index and emit a syntax error if the `Name` is already bound in the
current scope at the point of the `global` statement. This has the
downside of separating the error from the `SemanticSyntaxChecker`, but I
plan to explore using this approach in the `SemanticSyntaxChecker`
itself as a follow-up. It seems like this may be a better approach for
ruff as well.

Test Plan
--

Updated all of the related mdtests to remove the TODOs (and add quotes I
forgot on the messages).

There is one remaining TODO, but it requires `nonlocal` support, which
isn't even incorporated into the `SemanticSyntaxChecker` yet.

---------

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>
Co-authored-by: Carl Meyer <carl@astral.sh>

crates/ty_python_semantic/src/semantic_index.rs

@@ -176,6 +176,9 @@ pub(crate) struct SemanticIndex<'db> {
     /// Map from the file-local [`FileScopeId`] to the salsa-ingredient [`ScopeId`].
     scope_ids_by_scope: IndexVec<FileScopeId, ScopeId<'db>>,
 
+    /// Map from the file-local [`FileScopeId`] to the set of explicit-global symbols it contains.
+    globals_by_scope: FxHashMap<FileScopeId, FxHashSet<ScopedSymbolId>>,
+
     /// Use-def map for each scope in this file.
     use_def_maps: IndexVec<FileScopeId, Arc<UseDefMap<'db>>>,
 
@@ -255,6 +258,16 @@ impl<'db> SemanticIndex<'db> {
         self.scope_ids_by_scope.iter().copied()
     }
 
+    pub(crate) fn symbol_is_global_in_scope(
+        &self,
+        symbol: ScopedSymbolId,
+        scope: FileScopeId,
+    ) -> bool {
+        self.globals_by_scope
+            .get(&scope)
+            .is_some_and(|globals| globals.contains(&symbol))
+    }
+
     /// Returns the id of the parent scope.
     pub(crate) fn parent_scope_id(&self, scope_id: FileScopeId) -> Option<FileScopeId> {
         let scope = self.scope(scope_id);