Revert "[CodeCompletion] Disable module system headers validation"

This reverts commit 9ff8bb9.

Author: rintaro

lib/IDE/CompletionInstance.cpp

@@ -328,11 +328,6 @@ 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;
-
   // FIXME: ASTScopeLookup doesn't support code completion yet.
   Invocation.disableASTScopeLookup();