In a previous change, https://reviews.llvm.org/D65043, I allowed users to
explicitly opt-in to formatting according to the C++20 standard. Should
a user choose to do so, the formatter would treat coroutine keywords
such as co_yield as keywords, whereas under the C++11 standard they
would be treated as identifiers.
However, this excludes a use case in which, for example, a user's codebase
is written using the C++17 standard, but with an explicit opt-in to
-fcoroutines-ts. This commit adds an option to clang-format to allow
formatting under an arbitrary standard, but with co_yield and friends
treated as keywords.