[clang] [Clang][AST] fix crash in mangle lambda expression (PR #78896)

[Qizhi Hu via cfe-commits]

================
@@ -0,0 +1,4 @@
+// RUN: %clang_cc1 -fsyntax-only -std=c++20 -verify %s
----------------
jcsxky wrote:

Sorry, I am afraid that I misunderstand root cause of this issue and still need more working on it.

https://github.com/llvm/llvm-project/pull/78896