SE-0022: Deprecate string-literal-as-selector in favor of #selector.

Introduce Fix-Its to aid migration from selectors spelled as string
literals ("foo:bar:", which is deprecated), as well as from
construction of Selector instances from string literals
(Selector("foo:bar"), which is still acceptable but not recommended),
to the #selector syntax. Jump through some hoops to disambiguate
method references if there are overloads:

    fixits.swift:51:7: warning: use of string literal for Objective-C
         selectors is deprecated; use '#selector' instead
      _ = "overloadedWithInt:" as Selector
          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
          #selector(Bar.overloaded(_:) as (Bar) -> (Int) -> ())

In the cases where we cannot provide a Fix-It to a #selector
expression, we wrap the string literal in a Selector(...) construction
to suppress the deprecation warning. These are also easily searchable
in the code base.

This also means we're doing more validation of the string literals
that go into Selector, i.e., that they are well-formed selectors and
that we know about some method that is @objc and has that
selector. We'll warn if either is untrue.

include/swift/Serialization/ModuleFile.h

@@ -553,6 +553,11 @@ public:
                          DeclName name,
                          SmallVectorImpl<ValueDecl*> &results);
 
+  /// Find all Objective-C methods with the given selector.
+  void lookupObjCMethods(
+         ObjCSelector selector,
+         SmallVectorImpl<AbstractFunctionDecl *> &results);
+
   /// Reports all link-time dependencies.
   void collectLinkLibraries(Module::LinkLibraryCallback callback) const;