K2: allow/disallow coercion-to-unit for callable references more precisely

Before this commit, K2 always applied coercion-to-unit for
callable references if expected type was Unit, and actual non-Unit.
However, this may not work in case when actual return type is
a type parameter and it must be inferred into Unit.
In this commit we started to disallow coercion-to-unit
for references with synthetic outer call (~ top-level in K1)
AND a type parameter as a return type (both should be true to disallow).
This provides better K1 consistency,
while still keeping some broken K1 cases working in K2.

See also added comment in CallableReferenceResolution.kt.

#KT-62565 Fixed

compiler/testData/diagnostics/tests/inference/callableReferences/conversionLastStatementInLambdaGeneric.fir.kt

@@ -0,0 +1,30 @@
+// DIAGNOSTICS: -UNCHECKED_CAST
+// This test is a copy of conversionLastStatementInLambda, but with generic functions
+// However works diffirently in K1
+
+fun main(b: Boolean) {
+    callWithLambda {
+        ::test1
+    }
+
+    callWithLambda {
+        if (b) ::test1 else ::test2
+    }
+
+    callWithLambda {
+        if (b) {
+            ::test1
+        } else {
+            ::test2
+        }
+    }
+
+    callWithLambda {
+        (::test1)
+    }
+}
+
+fun <T> test1(): T = "" as T
+fun <T> test2(): T = "" as T
+
+fun callWithLambda(action: () -> () -> Unit) {}

compiler/testData/diagnostics/tests/inference/callableReferences/conversionLastStatementInLambdaGeneric.kt

@@ -0,0 +1,30 @@
+// DIAGNOSTICS: -UNCHECKED_CAST
+// This test is a copy of conversionLastStatementInLambda, but with generic functions
+// However works diffirently in K1
+
+fun main(b: Boolean) {
+    callWithLambda {
+        <!NEW_INFERENCE_NO_INFORMATION_FOR_PARAMETER!>::test1<!>
+    }
+
+    callWithLambda {
+        if (b) <!NEW_INFERENCE_NO_INFORMATION_FOR_PARAMETER!>::test1<!> else <!NEW_INFERENCE_NO_INFORMATION_FOR_PARAMETER!>::test2<!>
+    }
+
+    callWithLambda {
+        if (b) {
+            ::test1
+        } else {
+            ::test2
+        }
+    }
+
+    callWithLambda {
+        (<!NEW_INFERENCE_NO_INFORMATION_FOR_PARAMETER!>::test1<!>)
+    }
+}
+
+fun <T> test1(): T = "" as T
+fun <T> test2(): T = "" as T
+
+fun callWithLambda(action: () -> () -> Unit) {}

compiler/testData/diagnostics/tests/inference/coercionToUnit/genericInApply.kt

@@ -0,0 +1,10 @@
+// FIR_IDENTICAL
+// WITH_STDLIB
+
+fun <N : Number> addNumber(n: N) = n
+
+fun foo() = 42
+
+fun test() {
+    foo().apply(::addNumber)
+}