[Caching] Do not serialize module deps when caching

There is no need to serialize module deps when caching is on because the
entire build command has already capture the entire dependency tree.
There is no need to encode the has for all dependencies and try to
validate them.

rdar://122914546

Author: cachemeifyoucan

lib/FrontendTool/FrontendTool.cpp

@@ -410,6 +410,13 @@ static bool buildModuleFromInterface(CompilerInstance &Instance) {
   bool IgnoreAdjacentModules = Instance.hasASTContext() &&
                                Instance.getASTContext().IgnoreAdjacentModules;
 
+  // When caching is enabled, the explicit module build dependencies are
+  // discovered by dependency scanner and the swiftmodule is already rebuilt
+  // ignoring candidate module. There is no need to serialized dependencies for
+  // validation purpose.
+  bool ShouldSerializeDeps =
+      !Instance.getInvocation().getCASOptions().EnableCaching;
+
   // If an explicit interface build was requested, bypass the creation of a new
   // sub-instance from the interface which will build it in a separate thread,
   // and isntead directly use the current \c Instance for compilation.
@@ -422,8 +429,7 @@ static bool buildModuleFromInterface(CompilerInstance &Instance) {
     return ModuleInterfaceLoader::buildExplicitSwiftModuleFromSwiftInterface(
         Instance, Invocation.getClangModuleCachePath(),
         FEOpts.BackupModuleInterfaceDir, PrebuiltCachePath, ABIPath, InputPath,
-        Invocation.getOutputFilename(),
-        /* shouldSerializeDeps */ true,
+        Invocation.getOutputFilename(), ShouldSerializeDeps,
         Invocation.getSearchPathOptions().CandidateCompiledModules);
 
   return ModuleInterfaceLoader::buildSwiftModuleFromSwiftInterface(