swift

ad1acd7f - Frontend: Allow -enable-experimental-feature to specify upcoming features.

Frontend: Allow -enable-experimental-feature to specify upcoming features. During the lifecycle of a feature, it may start as an experimental feature and then graduate to become an upcoming feature. To preserve compatibility with projects that adopted the feature when it was experimental, `-enable-experimental-feature` ought to be able to enable upcoming features, too. Since projects may use `-enable-experimental-feature` for compatibility with an older toolchain that does not have the feature as an upcoming feature, there is no warning when the flag is used to enable an upcoming feature. Note that if the semantics of a feature change when it graduates from experimental to upcoming, then the feature must be renamed so that projects using the experimental feature have an opportunity opt-in to the new semantics of the upcoming feature. Resolves rdar://134276783.