Before:
module file's minimum deployment target is iOS 12.0:
/path/to/FooKit.swiftmodule
After:
compiling for iOS 11.0, but module 'FooKit' has a minimum deployment
target of iOS 12.0: /path/to/FooKit.swiftmodule
Also tweak the "incompatible target" error to include the module name.
rdar://problem/35546499