[Omit needless words] Split first selector piece into base name/first argument label.

include/swift/Basic/StringExtras.h

@@ -319,6 +319,9 @@ enum class OmissionTypeFlags {
 
   /// Whether this parameter is of some Boolean type.
   Boolean = 0x02,
+
+  /// Whether this parameter is of some function/block type.
+  Function = 0x04,
 };
 
 /// Options that described omitted types.
@@ -370,6 +373,11 @@ struct OmissionTypeName {
     return Options.contains(OmissionTypeFlags::Boolean);
   }
 
+  /// Whether this type is a function/block type.
+  bool isFunction() const {
+    return Options.contains(OmissionTypeFlags::Function);
+  }
+
   /// Determine whether the type name is empty.
   bool empty() const { return Name.empty(); }