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/functionLiterals/assignmentOperationInLambdaWithExpectedType.fir.kt

@@ -11,5 +11,5 @@ fun test(bal: Array<Int>) {
 
     val e: Unit = run { bar += 4 }
 
-    val f: Int = <!INITIALIZER_TYPE_MISMATCH!>run <!ARGUMENT_TYPE_MISMATCH!>{ bar += 4 }<!><!>
+    val f: Int = run { <!ARGUMENT_TYPE_MISMATCH!>bar += 4<!> }
 }