[Driver] Main compiler tells frontend to -emit-loaded-module-trace.

include/swift/Driver/Compilation.h

@@ -146,6 +146,10 @@ private:
   /// detail than ShowIncrementalBuildDecisions.
   bool ShowJobLifecycle = false;
 
+  /// When true, some frontend job has requested permission to pass
+  /// -emit-loaded-module-trace, so no other job needs to do it.
+  bool PassedEmitLoadedModuleTraceToFrontendJob = false;
+
   static const Job *unwrap(const std::unique_ptr<const Job> &p) {
     return p.get();
   }
@@ -232,6 +236,22 @@ public:
   /// -2 indicates that one of the Compilation's Jobs crashed during execution
   int performJobs();
 
+  /// Returns whether the callee is permitted to pass -emit-loaded-module-trace
+  /// to a frontend job.
+  ///
+  /// This only returns true once, because only one job should pass that
+  /// argument.
+  bool requestPermissionForFrontendToEmitLoadedModuleTrace() {
+    if (PassedEmitLoadedModuleTraceToFrontendJob)
+      // Someone else has already done it!
+      return false;
+    else {
+      // We're the first and only (to execute this path).
+      PassedEmitLoadedModuleTraceToFrontendJob = true;
+      return true;
+    }
+  }
+
 private:
   /// \brief Perform all jobs.
   ///