Add frontend flags for staging cross-import overlays

Author: beccadax

include/swift/Basic/LangOptions.h

@@ -109,6 +109,9 @@ namespace swift {
     /// human-readable string.
     bool EnableConcisePoundFile = false;
 
+    /// Detect and automatically import modules' cross-import overlays.
+    bool EnableCrossImportOverlays = false;
+
     ///
     /// Support for alternate usage modes
     ///

include/swift/Option/FrontendOptions.td

@@ -126,6 +126,11 @@ def verify_syntax_tree : Flag<["-"], "verify-syntax-tree">,
 def show_diagnostics_after_fatal : Flag<["-"], "show-diagnostics-after-fatal">,
   HelpText<"Keep emitting subsequent diagnostics after a fatal error">;
 
+def enable_cross_import_overlays : Flag<["-"], "enable-cross-import-overlays">,
+  HelpText<"Automatically import declared cross-import overlays.">;
+def disable_cross_import_overlays : Flag<["-"], "disable-cross-import-overlays">,
+  HelpText<"Do not automatically import declared cross-import overlays.">;
+
 def enable_descriptive_diagnostics : Flag<["-"], "enable-descriptive-diagnostics">,
   HelpText<"Show descriptive diagnostic information, if available.">;
 

lib/Frontend/CompilerInvocation.cpp

@@ -543,6 +543,11 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
   Opts.EnableConcisePoundFile =
       Args.hasArg(OPT_enable_experimental_concise_pound_file);
 
+  Opts.EnableCrossImportOverlays =
+      Args.hasFlag(OPT_enable_cross_import_overlays,
+                   OPT_disable_cross_import_overlays,
+                   false);
+
   llvm::Triple Target = Opts.Target;
   StringRef TargetArg;
   if (const Arg *A = Args.getLastArg(OPT_target)) {