[clang] [lldb] [Clang] Improve Sema diagnostic performance for __builtin_counted_by_ref (PR #116719)

[via cfe-commits]

Sirraide wrote:

> Could you point to a place in the code where it creates a placeholder?

I mean, e.g. `CheckPointerToMemberOperands()` can return `BoundMemberTy` as the type of a `.*` expression, and `CreateBuiltinMatrixSubscriptExpr()` creates a `MatrixSubscriptExpr` with type `IncompleteMatrixIdxTy` if that’s what you’re asking.

https://github.com/llvm/llvm-project/pull/116719