[ty] perform type narrowing for places marked `global` too (#19381)

Fixes https://github.com/astral-sh/ty/issues/311.

crates/ty_python_semantic/resources/mdtest/scopes/global.md

@@ -71,6 +71,19 @@ def f():
     reveal_type(x)  # revealed: Literal[1]
 ```
 
+Same for an `if` statement:
+
+```py
+x: int | None
+
+def f():
+    # The `global` keyword isn't necessary here, but this is testing that it doesn't get in the way
+    # of narrowing.
+    global x
+    if x == 1:
+        y: int = x  # allowed, because x cannot be None in this branch
+```
+
 ## `nonlocal` and `global`
 
 A binding cannot be both `nonlocal` and `global`. This should emit a semantic syntax error. CPython

crates/ty_python_semantic/src/types/infer.rs

@@ -6377,7 +6377,13 @@ impl<'db, 'ast> TypeInferenceBuilder<'db, 'ast> {
             if let Some(name) = expr.as_name() {
                 if let Some(symbol_id) = place_table.place_id_by_name(name) {
                     if self.skip_non_global_scopes(file_scope_id, symbol_id) {
-                        return global_symbol(self.db(), self.file(), name);
+                        return global_symbol(self.db(), self.file(), name).map_type(|ty| {
+                            self.narrow_place_with_applicable_constraints(
+                                expr,
+                                ty,
+                                &constraint_keys,
+                            )
+                        });
                     }
                     is_nonlocal_binding = self
                         .index