Added -enable-experimental-defined-lifetimes.

The new flag will guard work on the experimental implementation of
tracking lexical lifetimes in SIL.

lib/Frontend/CompilerInvocation.cpp

@@ -422,6 +422,9 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
   Opts.EnableExperimentalConcurrency |=
     Args.hasArg(OPT_enable_experimental_concurrency);
 
+  Opts.EnableExperimentalDefinedLifetimes |=
+      Args.hasArg(OPT_enable_experimental_defined_lifetimes);
+
   Opts.EnableExperimentalNamedOpaqueTypes |=
       Args.hasArg(OPT_enable_experimental_named_opaque_types);