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

[Benjamin Maxwell via cfe-commits]

MacDue wrote:

> Please add some tests for lambdas with streaming attributes, for example:
> 
> ```
> void f() { auto x = []__arm_locally_streaming {}; x(); }
> ```

Done :+1: 

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