Revert "[CodeCompletion] Disable module system headers validation"

This reverts commit ec70a40.

Author: rintaro

lib/IDE/CompletionInstance.cpp

@@ -362,11 +362,6 @@ bool swift::ide::CompletionInstance::performOperation(
   // source text. That breaks an invariant of syntax tree building.
   Invocation.getLangOptions().BuildSyntaxTree = false;
 
-  // This validation may call stat(2) many times. Disable it to prevent
-  // performance regression.
-  Invocation.getSearchPathOptions().DisableModulesValidateSystemDependencies =
-      true;
-
   // Since caching uses the interface hash, and since per type fingerprints
   // weaken that hash, disable them here:
   Invocation.getLangOptions().EnableTypeFingerprints = false;