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

[via cfe-commits]

Sirraide wrote:

> the argument clause "(...)" is stored as its source text

One thing I did notice is that you’re calling `Lexer::getSourceText()` for this: how does this interact w/ macros and comments? I’m assuming it just retains those? Do we want that?

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