[LLDB] Allow one-line summaries in the presence of synthetic child providers (#184926)
This is driven by the Swift language. In Swift many data types such as
Int, and String are structs, and LLDB provides summary formatters and
synthetic child providers for them. For String, for example, a summary
formatter pulls out the string data from the implementation, while a
synthetic child provider hides the implementation details from users, so
strings don't expand their children.
rdar://171646109