[clang] [clang-repl] Reset the implicit instantiations of a failed input (PR #226253)

[Emery Conrad via cfe-commits]

conrade-ctc wrote:

> Please take a look how this is done in cling in a very non-invasive way to do that.

Thanks. cling gets the instantiations from the existing consumer calls, so Sema needs no change. On unload, it removes the new specializations from their templates. This PR resets them in place instead.

#223305 covers the same ground, including class templates. I think a second mechanism would collide. Which do you prefer?

1. The cling way, in `ASTDeclUnmerger` as in #218149.
2. Fold this into #223305. This PR then keeps only the parts that any rollback needs: the two `FIXME: Notify listener` fixes and the `VarTemplateSpecializationDecl` setter.

Either way, the tests for Eli's poisoned cases can move to the change that lands.


https://github.com/llvm/llvm-project/pull/226253