Added -enable-experimental-defined-lifetimes.

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

Author: nate-chandler

include/swift/Basic/LangOptions.h

@@ -298,6 +298,9 @@ namespace swift {
     /// Enable experimental concurrency model.
     bool EnableExperimentalConcurrency = false;
 
+    /// Enable experimental support for emitting defined borrow scopes.
+    bool EnableExperimentalDefinedLifetimes = false;
+
     /// Enable experimental support for named opaque result types, e.g.
     /// `func f() -> <T> T`.
     bool EnableExperimentalNamedOpaqueTypes = false;

include/swift/Option/FrontendOptions.td

@@ -243,6 +243,10 @@ def enable_experimental_concurrency :
   Flag<["-"], "enable-experimental-concurrency">,
   HelpText<"Enable experimental concurrency model">;
 
+def enable_experimental_defined_lifetimes :
+  Flag<["-"], "enable-experimental-defined-lifetimes">,
+  HelpText<"Enable experimental defined lifetimes">;
+
 def enable_experimental_distributed :
   Flag<["-"], "enable-experimental-distributed">,
   HelpText<"Enable experimental 'distributed' actors and functions">;

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);