Generate IMPLICIT_NOTNULL for (nullability flexible type) -> (non-null type) implicit conversion.

compiler/testData/ir/irText/implicitCastOnPlatformType.kt

@@ -0,0 +1,2 @@
+fun test(): String =
+        System.getProperty("test")

compiler/testData/ir/irText/implicitCastOnPlatformType.txt

@@ -0,0 +1,8 @@
+IrFile /implicitCastOnPlatformType.kt
+  IrFunction public fun test(): kotlin.String
+    IrExpressionBody
+      BLOCK type=<no-type> hasResult=false operator=null
+        RETURN type=<no-type>
+          UNARY_OP operator=IMPLICIT_NOTNULL type=kotlin.String related=null
+            CALL .getProperty type=kotlin.String! operator=null
+              p0: LITERAL String type=kotlin.String value='test'