[FIR] Rename FirAnnotation.classId to unexpandedClassId

This makes the eye catch each such thing in the source.
OptIn isn't that catchy, plus it may be declared
for the whole file, so noticing may be harder

compiler/fir/tree/src/org/jetbrains/kotlin/fir/expressions/FirExpressionUtil.kt

@@ -32,7 +32,7 @@ import org.jetbrains.kotlin.types.ConstantValueKind
 inline val FirAnnotation.coneClassLikeType: ConeClassLikeType?
     get() = ((annotationTypeRef as? FirResolvedTypeRef)?.type as? ConeClassLikeType)
 
-inline val FirAnnotation.classId: ClassId?
+inline val FirAnnotation.unexpandedClassId: ClassId?
     get() = coneClassLikeType?.lookupTag?.classId
 
 fun <T> buildConstOrErrorExpression(source: KtSourceElement?, kind: ConstantValueKind<T>, value: T?, diagnostic: ConeDiagnostic): FirExpression =