[clang] [clang-tools-extra] [clang] Retain unrecognized C++ [[attributes]] in the AST (PR #209224)

[Matheus Izvekov via cfe-commits]

mizvekov wrote:

The only I problematic bit I can see about these attributes in templated declarations is if they have arguments, we don't know if they are types or expressions and so would need instantiation.

https://github.com/llvm/llvm-project/pull/209224