swift234

[Sema] Make it possible to suppress warnings about Selector("foo").

When the selector named by Selector("foo") does not map to a known
Objective-C method, allow one to suppress the warning by wrapping the
string literal in an extra set of parentheses, e.g.,

  Selector(("foo"))

Suggest this via a Fix-It on a note so it's discoverable. Addresses
rdar://problem/24791200.