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

[via cfe-commits]

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@@ -0,0 +1,4 @@
+// RUN: %clang_cc1 -fsyntax-only -std=c++20 -verify %s
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cor3ntin wrote:

`-fsyntax-only` is suspicious, i would prefer a CodeGenTest that checks the mangle name
Maybe moving the test to `test/CodeGenCXX/mangle-lambdas.cpp ` (or one of the other similar test in this folder) would be a good idea

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