FIR: assume a lambda returns Unit if it ends with a non-expression

While it is theoretically useful to know that `{ while(true) {} }`
returns Nothing, CFG node deadness is not precise enough to do that: if
the entire lambda is dead, it's no longer possible to find out whether
the loop is terminating. Besides, `while (true)` and `if (true)` are
pretty much the only constructs like that anyway.

Note that this commit does not affect resolution for lambdas that end in
a Nothing-returning expression, e.g. `throw`.

compiler/testData/diagnostics/tests/inference/coercionToUnit/coersionWithAnonymousFunctionsAndUnresolved.fir.kt

@@ -65,7 +65,7 @@ fun testParameter() {
     takeFnToParameter(fun(): Unit { return Unit })
     takeFnToParameter(fun() { if (true) return })
     takeFnToParameter(fun() { if (true) return Unit })
-    <!NEW_INFERENCE_NO_INFORMATION_FOR_PARAMETER!>takeFnToParameter<!>(fun() = <!UNRESOLVED_REFERENCE!>unresolved<!>())
+    takeFnToParameter(fun() = <!UNRESOLVED_REFERENCE!>unresolved<!>())
     takeFnToParameter(fun() { <!UNRESOLVED_REFERENCE!>unresolved<!>() })
     takeFnToParameter(fun(): Unit { <!UNRESOLVED_REFERENCE!>unresolved<!>() })
     takeFnToParameter(fun() { return <!UNRESOLVED_REFERENCE!>unresolved<!>() })