bpo-43244: Remove Yield macro from pycore_ast.h (GH-25243)

* pycore_ast.h no longer defines the Yield macro.
* Fix a compiler warning on Windows: "warning C4005: 'Yield': macro
  redefinition".
* Python-ast.c now defines directly functions with their real
  _Py_xxx() name, rather than xxx().
* Remove "#undef Yield" in C files including pycore_ast.h.
diff --git a/Python/ast_opt.c b/Python/ast_opt.c
index 0310466..46dba76 100644
--- a/Python/ast_opt.c
+++ b/Python/ast_opt.c
@@ -451,8 +451,8 @@ astfold_body(asdl_stmt_seq *stmts, PyArena *ctx_, _PyASTOptimizeState *state)
             return 0;
         }
         asdl_seq_SET(values, 0, st->v.Expr.value);
-        expr_ty expr = JoinedStr(values, st->lineno, st->col_offset,
-                                 st->end_lineno, st->end_col_offset, ctx_);
+        expr_ty expr = _Py_JoinedStr(values, st->lineno, st->col_offset,
+                                     st->end_lineno, st->end_col_offset, ctx_);
         if (!expr) {
             return 0;
         }