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/tests/inference/smartCastFork.fir.kt

@@ -0,0 +1,25 @@
+// SKIP_TXT
+// !DIAGNOSTICS: -UNUSED_VARIABLE
+
+interface A<E> {
+    fun foo(): E
+}
+
+interface B : A<Int>
+interface C : A<Long>
+
+fun <T> bar(a: A<T>, w: T) {
+    baz(a, w) // OK in FE1.0
+
+    if (a is B) {
+        baz(a, 1) // OK in FE1.0
+        baz(a, w) // Type mismatch: Required Int, but found E
+        <!INFERENCE_UNSUCCESSFUL_FORK("it(B & A<T>) <: A<TypeVariable(F)>")!>baz<!>(a, "")
+    }
+
+    if (a is B || a is C) {
+        baz(a, w) // OK in FE 1.0 (Smart cast doesn't work), fail at FIR: it infers F to `LBU(Int, Long)` and then `w` is considered inapplicable
+    }
+}
+
+fun <F> baz(a: A<F>, f: F) {}

compiler/testData/diagnostics/tests/inference/smartCastFork.kt

@@ -0,0 +1,25 @@
+// SKIP_TXT
+// !DIAGNOSTICS: -UNUSED_VARIABLE
+
+interface A<E> {
+    fun foo(): E
+}
+
+interface B : A<Int>
+interface C : A<Long>
+
+fun <T> bar(a: A<T>, w: T) {
+    baz(a, w) // OK in FE1.0
+
+    if (a is B) {
+        baz(<!DEBUG_INFO_SMARTCAST!>a<!>, 1) // OK in FE1.0
+        baz(<!DEBUG_INFO_SMARTCAST!>a<!>, <!TYPE_MISMATCH!>w<!>) // Type mismatch: Required Int, but found E
+        baz(<!DEBUG_INFO_SMARTCAST!>a<!>, <!TYPE_MISMATCH!>""<!>)
+    }
+
+    if (a is B || a is C) {
+        baz(a, w) // OK in FE 1.0 (Smart cast doesn't work), fail at FIR: it infers F to `LBU(Int, Long)` and then `w` is considered inapplicable
+    }
+}
+
+fun <F> baz(a: A<F>, f: F) {}

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)
+}