Add an opt-in warning to complain about needless words in declarations.

The new option -Womit-needless-words finds places where names are
redundant with type information, producing warnings and Fix-Its to
shorten the names. Part of rdar://problem/22232287, to help bring
the same heuristics we're applying in the Clang importer to the user's
Swift code.

Swift SVN r31234

lib/Frontend/CompilerInvocation.cpp

@@ -669,6 +669,7 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
   if (Opts.DebuggerSupport)
     Opts.EnableDollarIdentifiers = true;
   Opts.Playground |= Args.hasArg(OPT_playground);
+  Opts.WarnOmitNeedlessWords = Args.hasArg(OPT_warn_omit_needless_words);
 
   Opts.EnableThrowWithoutTry |= Args.hasArg(OPT_enable_throw_without_try);