Optimize KtFile::isScipt/getScript when file is not kts

`isScript` gets invoked for each call
(see org.jetbrains.kotlin.resolve.calls.CandidateResolver#checkOuterClassMemberIsAccessible)

And currently it may work for O(|top-level-declarations|),
that is rather slow e.g. for native stubs for gtk

 #KT-17562 Fixed

compiler/frontend/src/org/jetbrains/kotlin/psi/KtFile.java

@@ -46,6 +46,7 @@ import java.util.List;
 public class KtFile extends PsiFileBase implements KtDeclarationContainer, KtAnnotated, KtElement, PsiClassOwner, PsiNamedElement {
 
     private final boolean isCompiled;
+    private Boolean isScript = null;
 
     public KtFile(FileViewProvider viewProvider, boolean compiled) {
         super(viewProvider, KotlinLanguage.INSTANCE);
@@ -183,9 +184,22 @@ public class KtFile extends PsiFileBase implements KtDeclarationContainer, KtAnn
     @Override
     public void setPackageName(String packageName) { }
 
+    @Override
+    public void clearCaches() {
+        super.clearCaches();
+        isScript = null;
+    }
+
     @Nullable
     public KtScript getScript() {
-        return PsiTreeUtil.getChildOfType(this, KtScript.class);
+        if (isScript != null && !isScript) return null;
+
+        KtScript result = PsiTreeUtil.getChildOfType(this, KtScript.class);
+        if (isScript == null) {
+            isScript = result != null;
+        }
+
+        return result;
     }
 
     public boolean isScript() {