NI: Support forking inference with heuristics

Mostly, it only affects FIR

It partially allows to consider several variance of constraints like
A<Int> & A<T> <: A<X_var> that are mostly brought by smart casts

^KT-49542 Fixed
^KT-50489 Relates

compiler/testData/diagnostics/testsWithStdLib/inference/plusAssignOnArrayList.kt

@@ -0,0 +1,9 @@
+// FIR_IDENTICAL
+// SKIP_TXT
+import java.util.*
+
+val a = ArrayList<String>()
+
+fun foo(l: List<String>) {
+    a.plusAssign(l)
+}