Add Real Enable/Disable Flags for Cross-Module Incremental Builds

In the legacy driver, these flags will merely be propagated to the
frontends to indicate that they should disable serialization of
incremental information in swift module files.

In the new driver, these flags control whether the Swift driver performs
an incremental build that is aware of metadata embedded in the module.

Kudos to David for coming up with our new marketing name: Incremental
Imports.

rdar://74363450

lib/FrontendTool/FrontendTool.cpp

@@ -1536,7 +1536,11 @@ static bool performCompileStepsPostSILGen(CompilerInstance &Instance,
 
     SerializationOptions serializationOpts =
         Invocation.computeSerializationOptions(outs, Instance.getMainModule());
-    if (serializationOpts.ExperimentalCrossModuleIncrementalInfo) {
+
+    const bool canEmitIncrementalInfoIntoModule =
+        !serializationOpts.DisableCrossModuleIncrementalInfo &&
+        (Action == FrontendOptions::ActionType::MergeModules);
+    if (canEmitIncrementalInfoIntoModule) {
       const auto alsoEmitDotFile =
           Instance.getInvocation()
               .getLangOptions()