The legacy `module.map` spelling of module map files was deprecated by llvm/llvm-project#75142 and clang expects to remove support for them in the future. Switch all tests to use the supported spelling.
Fixes rdar://128431478.
Rather than checking the underlying ObjectiveC decl, use the presence of
`OptionalAttr` to determine if a requirement is optional instead. This
is already added by the importer when necessary. An added benefit here
is that this also works for optional requirements defined in Swift (ie.
`@objc optional ...`).
