[lldb] Use summary string for ObjectiveC.Selector (NFC)

[kastiglione]

Replace the ObjC_Selector_SummaryProvider function with the equivalent summary string.

[kastiglione]

Going to add a test.

[kastiglione]

@jimingham that's about what I was thinking too.

[jimingham]

If you knew that the category you were adding the summary to only supported the swift language, then you could force the EscapeStyle to swift on the summary string formatters added to it. But those formatters are currently in the "runtime synthetics" category which is both swift & ObjC.