[Diagnostics] Adding optional note specific to keypath application base diagnostics

Author: LucianoPAlmeida

include/swift/AST/DiagnosticsSema.def

@@ -1077,7 +1077,9 @@ NOTE(optional_base_chain,none,
      "base values", (DeclNameRef))
 NOTE(optional_base_remove_optional_for_keypath_root, none,
      "use unwrapped type %0 as key path root", (Type))
-
+NOTE(optional_keypath_application_base, none,
+     "use '?' to access key path subscript only for non-'nil' base values", ())
+    
 ERROR(missing_unwrap_optional_try,none,
       "value of optional type %0 not unwrapped; did you mean to use 'try!' "
       "or chain with '?'?",

localization/diagnostics/en.yaml

@@ -4292,6 +4292,10 @@
   msg: >-
     use unwrapped type %0 as key path root
 
+- id: optional_keypath_application_base 
+  msg: >-
+    use '?' to access key path subscript only for non-'nil' base values
+
 - id: missing_unwrap_optional_try
   msg: >-
     value of optional type %0 not unwrapped; did you mean to use 'try!' or