Downgrade the new error in https://github.com/swiftlang/swift/pull/84244 to a
warning.
The PR smoke test build is using a different build system that is failing to
pass `-swift-version` arguments when building various stdlib modules. That
needs to be fixed, but for now we also need to unblock CI.
If a `.swiftinterface` file does not include an explicit `-language-mode`
option (or its predecessor `swift-version`) and needs to be built as a
dependency of a client compilation, then the invocation to build the module
from interface would end up inheriting the language mode that the client code
is built with. This can result in spurious type checking diagnostics or even
mis-compilation. To ensure that a module interface is always built using the
language mode that its source code was originally built with, require an
explicit `-language-mode` option when emitting swiftinterface files.
Resolves rdar://145168219.
