Add error-level OptIn on safeAs, cast and assertedCast functions in addToStdlib

Usage of this function is unsafe because it does not have native compiler
  support. This means that compiler won't report UNCHECKED_CAST,
  CAST_NEVER_SUCCEED or similar diagnostics in case of error cast
  (which can happen immediately or after some refactoring of class hierarchy)

libraries/reflect/api/build.gradle

@@ -52,6 +52,7 @@ compileKotlin {
                 "-Xno-optimized-callable-references",
                 "-Xno-kotlin-nothing-value-exception",
                 "-opt-in=kotlin.RequiresOptIn",
+                "-opt-in=org.jetbrains.kotlin.utils.addToStdlib.UnsafeCastFunction",
                 "-Xno-new-java-annotation-targets",
         ]
         moduleName = "kotlin-reflection"