[PSI] Increment modification stamp on adding imports to 'KtCodeFragment'

LL API incremental analysis depends on modification stamps for files and
declarations. Adding an import directive would modify a modification
stamp of an ordinary KtFile. The same should apply to a KtCodeFragment.

compiler/psi/src/org/jetbrains/kotlin/psi/KtCodeFragment.kt

@@ -145,6 +145,7 @@ abstract class KtCodeFragment(
         if (contextFile != null) {
             if (contextFile.importDirectives.find { it.text == import } == null) {
                 imports.add(import)
+                clearCaches() // Increment the modification stamp
             }
         }
     }