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/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)
+}