When a module has been imported `@preconcurrency` in source, when it is printed
in a `swiftinterface` file it should be printed along with the attribute to
ensure that type checking of the module's public declarations behaves
consistently.
This fix is a little unsatisfying because it adds another a linear scan over
all imports in the source for each printed import. This should be improved, but
it can be done later.
Resolves rdar://136857313.
