[semantic-arc-opts] Create a new pass called semantic arc opts.

I am going to run it very early and use it to ensure that extra copies due to my
refactoring of SILGenPattern do not cause COW copies to be introduced.

For now, it does a very simple optimization, namely, it eliminates a copy_value,
with only a destroy_value user on a guaranteed parameter.

It is now disabled behind a flag.

lib/Frontend/CompilerInvocation.cpp

@@ -1296,6 +1296,8 @@ static bool ParseSILArgs(SILOptions &Opts, ArgList &Args,
   Opts.EnableSILOwnership |= Args.hasArg(OPT_enable_sil_ownership);
   Opts.AssumeUnqualifiedOwnershipWhenParsing
     |= Args.hasArg(OPT_assume_parsing_unqualified_ownership_sil);
+  Opts.EnableMandatorySemanticARCOpts |=
+      Args.hasArg(OPT_enable_mandatory_semantic_arc_opts);
 
   if (Args.hasArg(OPT_debug_on_sil)) {
     // Derive the name of the SIL file for debugging from