- Fix code generation for inlined String::plus

#KT-10131 Fixed.
- Make extra factory methods in IntrinsicMethod protected.
This ensures that any of them can be properly overridden.

compiler/testData/codegen/boxWithStdlib/intrinsics/kt10131.kt

@@ -0,0 +1,2 @@
+fun box(): String =
+        listOf('O', 'K').fold("", String::plus)

compiler/testData/codegen/boxWithStdlib/intrinsics/kt10131a.kt

@@ -0,0 +1,2 @@
+fun box(): String =
+        charArrayOf('O', 'K').fold("", String::plus)