[clang] Revert "[Clang] disallow selectany on non-global-variable declarations" (PR #196511)

[Martin Storsjö via cfe-commits]

mstorsjo wrote:

> Reverts #189641

When reverting, I usually appreciate a word or two here about why it is being reverted. (Did it cause crashes, etc) The reason here is found in the PR link of course, but having at least one sentence about it in the revert as well usually makes it much easier for anybody looking at the revert commit in git afterwards.

https://github.com/llvm/llvm-project/pull/196511