[clang] [AST] Teach TextNodeDumper to print the "implicit" bit for coroutine AST nodes (PR #77311)

[Haojian Wu via cfe-commits]

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@@ -0,0 +1,46 @@
+// RUN: %clang_cc1 -triple x86_64-unknown-unknown %s -std=c++20 \
+// RUN:    -fsyntax-only -ast-dump -ast-dump-filter test | FileCheck %s
+
+#include "Inputs/std-coroutine.h"
+
+using namespace std;
+
+struct Task {
+  struct promise_type {
+    std::suspend_always initial_suspend() { return {}; }
+    Task get_return_object() {
+      return std::coroutine_handle<promise_type>::from_promise(*this);
+    }
+    std::suspend_always final_suspend() noexcept { return {}; }
+    std::suspend_always return_void() { return {}; }
+    void unhandled_exception() {}
+
+    auto await_transform(int s) {
+      struct awaiter {
+        promise_type *promise;
+        bool await_ready() { return true; }
+        int await_resume() { return 1; }
+        void await_suspend(std::coroutine_handle<>) {}
+      };
+
+      return awaiter{this};
+    }
+  };
+
+  Task(std::coroutine_handle<promise_type> promise);
+
+  std::coroutine_handle<promise_type> handle;
+};
+
+// Verify the implicit AST nodes for coroutines.
+Task test()  {
+  co_await 1;
+}
+// CHECK:        |-DeclStmt {{.*}}
+// CHECK-NEXT:   | `-VarDecl {{.*}} implicit used __promise
+// CHECK-NEXT:   |   `-CXXConstructExpr {{.*}}
+// CHECK-NEXT:   |-ExprWithCleanups {{.*}} 'void'
+// CHECK-NEXT:   | `-CoawaitExpr {{.*}} 'void' implicit
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hokein wrote:

> Is this node implicit for initial suspend, for a call to await_transform or both? 

This implicit node is just for initial suspend, it is the `InitSuspend` substmt of the `CoroutineBodyStmt`.

Added some more tests and matches for the written `co_await` and `co_return` statements.

https://github.com/llvm/llvm-project/pull/77311