[clang] [clang][sema][wat] peek through expansion statements to try and fix weird decl problems (PR #211745)

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Sirraide wrote:

Seeing this comment, could you add this a sa test:
```c++
void foo() {
  template for (auto x : {1, 2}) {
    template for (auto x : {1, 2}) { decltype(x) bar(); }
  }
}
```

https://github.com/llvm/llvm-project/pull/211745