When `-warn-on-potentially-unavailable-enum-case` was introduced, the build
system was required to invoke `swift-frontend` at artificially low deployment
targets when emitting `.swiftinterface` files for legacy architectures. Because
the deployment target was low, some availability diagnostics needed to be
de-fanged in order to allow module interface emission to succeed. Today, the
build system is able to use the correct deployment target when emitting module
interfaces and the `-warn-on-potentially-unavailable-enum-case` is superfluous,
so deprecate it.
Resolves rdar://114092047
There are two pieces here:
- A -warn-on-potentially-unavailable-enum-case flag is passed down by
the driver when *producing* a swiftinterface
- When *consuming* a swiftinterface, also enable this behavior
Part of rdar://problem/78306593.
