[SourceKit] Disable module system headers validation

in all SourceKit requests.
This validation may call many stat(2). Since we don't expect system files
are edited. Disable it for SourceKit requests. Even if they are edited,
manual builds can validates and updates them.

rdar://problem/58550697

lib/IDE/CompletionInstance.cpp

@@ -324,11 +324,6 @@ 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();