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

[Oleksandr Tarasiuk via cfe-commits]

a-tarasyuk wrote:

@mstorsjo @SquallATF, Thanks for reporting. To clarify, these changes were made after the spec clarification at https://github.com/llvm/llvm-project/pull/189641#discussion_r3146591253. As I understand it, the current behavior aligns with the clarified `selectany` rules. If we want to restore the previous, non-conforming behavior for compatibility with existing code, I can prepare a revert.

/cc @AaronBallman 

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