stdlib: Remove redundant checks from String.lastIndexOf
stdlib: Remove redundant checks from String.lastIndexOf

Kotlin/JVM allows a user to specify a start index greater than
a string length in String.lastIndexOf method. This patch
modifies the Kotlin/Native implementation according to this
behaviour. It also removes other redundant checks.

JetBrains/kotlin-native,JetBrains/kotlin-native,JuliusKunze/kotlin-native,jiaminglu/kotlin-native,JetBrains/kotlin-native,wiltonlazary/kotlin-native,jiaminglu/kotlin-native,JetBrains/kotlin-native,jiaminglu/kotlin-native,jiaminglu/kotlin-native,JuliusKunze/kotlin-native,jiaminglu/kotlin-native,wiltonlazary/kotlin-native,jiaminglu/kotlin-native,JuliusKunze/kotlin-native,JetBrains/kotlin-native,JuliusKunze/kotlin-native,JetBrains/kotlin-native,JetBrains/kotlin-native,JuliusKunze/kotlin-native,JetBrains/kotlin-native,jiaminglu/kotlin-native,JetBrains/kotlin-native,jiaminglu/kotlin-native,wiltonlazary/kotlin-native,wiltonlazary/kotlin-native,JuliusKunze/kotlin-native,JuliusKunze/kotlin-native,wiltonlazary/kotlin-native,JuliusKunze/kotlin-native