closes bpo-31902: Fix the col_offset attribute for ast.Async* nodes to point to the "async" keyword. (GH-4175)

Previously, col_offset points to the keyword after "async".
diff --git a/Python/ast.c b/Python/ast.c
index a91c075..ef2c858 100644
--- a/Python/ast.c
+++ b/Python/ast.c
@@ -600,8 +600,8 @@
 static expr_ty ast_for_testlist(struct compiling *, const node *);
 static stmt_ty ast_for_classdef(struct compiling *, const node *, asdl_seq *);
 
-static stmt_ty ast_for_with_stmt(struct compiling *, const node *, int);
-static stmt_ty ast_for_for_stmt(struct compiling *, const node *, int);
+static stmt_ty ast_for_with_stmt(struct compiling *, const node *, bool);
+static stmt_ty ast_for_for_stmt(struct compiling *, const node *, bool);
 
 /* Note different signature for ast_for_call */
 static expr_ty ast_for_call(struct compiling *, const node *, expr_ty, bool);
@@ -1569,10 +1569,11 @@
 }
 
 static stmt_ty
-ast_for_funcdef_impl(struct compiling *c, const node *n,
-                     asdl_seq *decorator_seq, int is_async)
+ast_for_funcdef_impl(struct compiling *c, const node *n0,
+                     asdl_seq *decorator_seq, bool is_async)
 {
     /* funcdef: 'def' NAME parameters ['->' test] ':' suite */
+    const node * const n = is_async ? CHILD(n0, 1) : n0;
     identifier name;
     arguments_ty args;
     asdl_seq *body;
@@ -1601,7 +1602,7 @@
 
     if (is_async)
         return AsyncFunctionDef(name, args, body, decorator_seq, returns,
-                                LINENO(n), n->n_col_offset, c->c_arena);
+                                LINENO(n), n0->n_col_offset, c->c_arena);
     else
         return FunctionDef(name, args, body, decorator_seq, returns,
                            LINENO(n), n->n_col_offset, c->c_arena);
@@ -1616,8 +1617,8 @@
     assert(strcmp(STR(CHILD(n, 0)), "async") == 0);
     REQ(CHILD(n, 1), funcdef);
 
-    return ast_for_funcdef_impl(c, CHILD(n, 1), decorator_seq,
-                                1 /* is_async */);
+    return ast_for_funcdef_impl(c, n, decorator_seq,
+                                true /* is_async */);
 }
 
 static stmt_ty
@@ -1625,7 +1626,7 @@
 {
     /* funcdef: 'def' NAME parameters ['->' test] ':' suite */
     return ast_for_funcdef_impl(c, n, decorator_seq,
-                                0 /* is_async */);
+                                false /* is_async */);
 }
 
 
@@ -1639,15 +1640,15 @@
 
     switch (TYPE(CHILD(n, 1))) {
         case funcdef:
-            return ast_for_funcdef_impl(c, CHILD(n, 1), NULL,
-                                        1 /* is_async */);
+            return ast_for_funcdef_impl(c, n, NULL,
+                                        true /* is_async */);
         case with_stmt:
-            return ast_for_with_stmt(c, CHILD(n, 1),
-                                     1 /* is_async */);
+            return ast_for_with_stmt(c, n,
+                                     true /* is_async */);
 
         case for_stmt:
-            return ast_for_for_stmt(c, CHILD(n, 1),
-                                    1 /* is_async */);
+            return ast_for_for_stmt(c, n,
+                                    true /* is_async */);
 
         default:
             PyErr_Format(PyExc_SystemError,
@@ -3681,8 +3682,9 @@
 }
 
 static stmt_ty
-ast_for_for_stmt(struct compiling *c, const node *n, int is_async)
+ast_for_for_stmt(struct compiling *c, const node *n0, bool is_async)
 {
+    const node * const n = is_async ? CHILD(n0, 1) : n0;
     asdl_seq *_target, *seq = NULL, *suite_seq;
     expr_ty expression;
     expr_ty target, first;
@@ -3717,7 +3719,7 @@
 
     if (is_async)
         return AsyncFor(target, expression, suite_seq, seq,
-                        LINENO(n), n->n_col_offset,
+                        LINENO(n), n0->n_col_offset,
                         c->c_arena);
     else
         return For(target, expression, suite_seq, seq,
@@ -3869,8 +3871,9 @@
 
 /* with_stmt: 'with' with_item (',' with_item)* ':' suite */
 static stmt_ty
-ast_for_with_stmt(struct compiling *c, const node *n, int is_async)
+ast_for_with_stmt(struct compiling *c, const node *n0, bool is_async)
 {
+    const node * const n = is_async ? CHILD(n0, 1) : n0;
     int i, n_items;
     asdl_seq *items, *body;
 
@@ -3892,7 +3895,7 @@
         return NULL;
 
     if (is_async)
-        return AsyncWith(items, body, LINENO(n), n->n_col_offset, c->c_arena);
+        return AsyncWith(items, body, LINENO(n), n0->n_col_offset, c->c_arena);
     else
         return With(items, body, LINENO(n), n->n_col_offset, c->c_arena);
 }