[clang] [Clang] disallow selectany on non-global-variable declarations (PR #189641)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> I think the main key word is external linkage in any case.

But then wouldn't it be accepted on:
```
struct T {
  SA static int i;
};
```
because that still has external linkage: https://godbolt.org/z/vafPxa4jh ?


https://github.com/llvm/llvm-project/pull/189641