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

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@@ -0,0 +1,4 @@
+// RUN: %clang_cc1 -fsyntax-only -std=c++20 -verify %s
+// expected-no-diagnostics
+
+auto ICE = [](auto a) { return [=]<decltype(a) b>() { return 1; }; };
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cor3ntin wrote:

Can you reference the issue number in the test?

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