[clang] Revert "[Clang][Sema] Handle invalid variable template specialization whose type depends on itself (#134522)" (PR #139539)

[Erich Keane via cfe-commits]

erichkeane wrote:

This would be quite disappointing to me.  As much as I strongly dislike that we don't build a standard library, this patch fixed a handful of bugs in a really nice manner.  

I'd vastly prefer if someone could prepare a 'compatibility' hack here. Even something as absolutely crazy as just skipping the args of `__primary_template_not_defined` when in the variable declaration for this exact thing, would probably be ok to me?  Is there anyone who can look into this? @zwuis ?

https://github.com/llvm/llvm-project/pull/139539