[semantic-arc] Add the -enable-sil-ownership flag and propagate its value to SILOptions.EnableSILOwnershipModel.

This will allow for semantic arc work to remain behind a flag and not affect
other in tree developers. More importantly it enables for bots to be setup with
this flag enabled.

rdar://28685236

include/swift/AST/SILOptions.h

@@ -113,6 +113,9 @@ public:
 
   /// The name of the SIL outputfile if compiled with SIL debugging (-gsil).
   std::string SILOutputFileNameForDebugging;
+
+  /// If set to true, compile with the SIL Ownership Model enabled.
+  bool EnableSILOwnership = false;
 };
 
 } // end namespace swift

include/swift/Option/FrontendOptions.td

@@ -227,6 +227,9 @@ def dump_clang_diagnostics : Flag<["-"], "dump-clang-diagnostics">,
 def emit_verbose_sil : Flag<["-"], "emit-verbose-sil">,
   HelpText<"Emit locations during SIL emission">;
 
+def enable_sil_ownership : Flag<["-"], "enable-sil-ownership">,
+  HelpText<"Enable the SIL Ownership Model">;
+
 def enable_experimental_property_behaviors :
   Flag<["-"], "enable-experimental-property-behaviors">,
   HelpText<"Enable experimental property behaviors">;

lib/Frontend/CompilerInvocation.cpp

@@ -1132,6 +1132,7 @@ static bool ParseSILArgs(SILOptions &Opts, ArgList &Args,
     Args.hasArg(OPT_enable_guaranteed_closure_contexts);
   Opts.DisableSILPartialApply |=
     Args.hasArg(OPT_disable_sil_partial_apply);
+  Opts.EnableSILOwnership |= Args.hasArg(OPT_enable_sil_ownership);
 
   if (Args.hasArg(OPT_debug_on_sil)) {
     // Derive the name of the SIL file for debugging from