[clang] [clang] Error on explicit specialization of lambda call operator (PR #84343)

[Shafik Yaghmour via cfe-commits]

shafik wrote:

Can we please get more descriptive summaries in the future. We should at a minimum state 1) what the underlying problem is 2) what the approach for fixing the problem is.

In this case the title describes the problem but it would be nice to have a summary of the fix as well. So folks reading the git log do not have to do an extra step to understand what the fix was at a high level.

https://github.com/llvm/llvm-project/pull/84343