[clang] [clang][SME] Account for C++ lambdas in SME builtin diagnostics (PR #124750)

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

Please add some tests for lambdas with streaming attributes, for example:

```
void f() { auto x = []__arm_locally_streaming {}; x(); }
```

https://github.com/llvm/llvm-project/pull/124750