Some string and minor optimizations

compiler/fir/tree/src/org/jetbrains/kotlin/fir/Utils.kt

@@ -70,12 +70,12 @@ fun FirSourceElement.getWholeQualifierSourceIfPossible(stepsToWholeQualifier: In
     return when (this) {
         is FirRealPsiSourceElement<*> -> {
             val qualifiersChain = generateSequence(psi) { it.parent }
-            val wholeQualifier = qualifiersChain.drop(stepsToWholeQualifier).first()
+            val wholeQualifier = qualifiersChain.elementAt(stepsToWholeQualifier)
             wholeQualifier.toFirPsiSourceElement() as FirRealPsiSourceElement
         }
         is FirLightSourceElement -> {
             val qualifiersChain = generateSequence(lighterASTNode) { treeStructure.getParent(it) }
-            val wholeQualifier = qualifiersChain.drop(stepsToWholeQualifier).first()
+            val wholeQualifier = qualifiersChain.elementAt(stepsToWholeQualifier)
             wholeQualifier.toFirLightSourceElement(
                 treeStructure,
                 startOffset = this.startOffset,

compiler/fir/tree/src/org/jetbrains/kotlin/fir/types/FirTypeUtils.kt

@@ -72,13 +72,13 @@ val FirFunctionTypeRef.parametersCount: Int
     else
         valueParameters.size
 
-const val EXTENSION_FUNCTION_ANNOTATION = "kotlin/ExtensionFunctionType"
+val EXTENSION_FUNCTION_ANNOTATION = ClassId.fromString("kotlin/ExtensionFunctionType")
 
 val FirAnnotationCall.isExtensionFunctionAnnotationCall: Boolean
     get() = (this as? FirAnnotationCall)?.let { annotationCall ->
         (annotationCall.annotationTypeRef as? FirResolvedTypeRef)?.let { typeRef ->
             (typeRef.type as? ConeClassLikeType)?.let {
-                it.lookupTag.classId.asString() == EXTENSION_FUNCTION_ANNOTATION
+                it.lookupTag.classId == EXTENSION_FUNCTION_ANNOTATION
             }
         }
     } == true