Make CMake build assume SWIFT_PATH_TO_SWIFT_SYNTAX_SOURCE location

Now we have to pass it explicitly which makes explicit cmake invocations, or using IDEs like CLion, VSCode which need invoke cmake to configure annoying. See: https://github.com/apple/swift/pull/62252

It would be better if the build can assume that swift syntax is located in the same directory as swift (i.e. `<swift-project>/swift/../swift-syntax`), that would make setup easier.