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/regressions/resolveCollectionLiteralInsideLambda.fir.kt

@@ -3,5 +3,5 @@
 fun foo(l: () -> Unit) {}
 fun bar(l: () -> String) {}
 
-val a = foo { <!ARGUMENT_TYPE_MISMATCH, UNSUPPORTED!>[]<!> }
+val a = foo { <!UNSUPPORTED!>[]<!> }
 val b = bar { <!ARGUMENT_TYPE_MISMATCH, UNSUPPORTED!>[]<!> }