Add initializers to postfix-expr code completions

Previously, the only way to get initializers was completing after the
name of the type:
    Foo#^complete_here^#
    Foo(#^or_here^#

And now it will also work in unadorned expressions:
    #^a_top_level_completion^#
    bar(a, #^walked_into_a_bar^#

Unfortunately, not all our clients handle this well yet, so it's
protected by a language option.
    -code-complete-inits-in-postfix-expr

Swift SVN r27275

include/swift/Basic/LangOptions.h

@@ -119,6 +119,10 @@ namespace swift {
     /// Should access control be respected?
     bool EnableAccessControl = true;
 
+    /// Whether to include initializers when code-completing a postfix
+    /// expression.
+    bool CodeCompleteInitsInPostfixExpr = false;
+
     /// Should we check the target OSs of serialized modules to see that they're
     /// new enough?
     bool EnableTargetOSChecking = true;

include/swift/Option/FrontendOptions.td

@@ -147,6 +147,9 @@ def disable_access_control : Flag<["-"], "disable-access-control">,
 def enable_access_control : Flag<["-"], "enable-access-control">,
   HelpText<"Respect access control restrictions">;
 
+def code_complete_inits_in_postfix_expr : Flag<["-"], "code-complete-inits-in-postfix-expr">,
+  HelpText<"Include initializers when completing a postfix expression">;
+
 def disable_diagnostic_passes : Flag<["-"], "disable-diagnostic-passes">,
   HelpText<"Don't run diagnostic passes">;