[clang] [ItaniumMangle] Fix `cp` versus `cl` call expression mangling for block scope (PR #114884)

[Hubert Tong via cfe-commits]

hubert-reinterpretcast wrote:

> Is it worth adding `ClangABICompat` support for this? (Do we think this will affect any real code, or is this more just formal correctness?)

The change was motivated by formal correctness (and the work led to the discovery of CWG 2946). I have some doubts that this change affects real code as the combination of `(func)(args ...)` in the signature of a lambda in an ODR context seems rare to me.

That said, if we don't add a `ClangABICompat` option now, then changing the meaning of `-fclang-abi-compat=19` in the future (if it is needed) could lead to a need for `-fclang-abi-compat=19-compat-under-20` or other such "compat compat" options.

https://github.com/llvm/llvm-project/pull/114884