[NI] Fix unit coercion

Consider following case:

fun foo(): Unit = run { "hello" }

Previously, NI would analyze lambda body without expected type, because
it is a type variable 'R' from 'run', which hasn't been fixed yet. This
leads to treating "hello" as lambda-return argument and adding bogus
'String' constraint on 'R', and, consequently, type mismatch.

Now, we peek into current constraint system and check if return-type of
lambda is type variable with upper-Unit constraint (which is exactly
condition for its body to be Unit-coerced). If so, then we provide
expected Unit-type for body explicitly, and the rest will be done
automatically (in particular, in aforementioned example "hello" wouldn't
be treated as lambda return argument).

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

@@ -0,0 +1,26 @@
+// !WITH_NEW_INFERENCE
+// !CHECK_TYPE
+
+fun noCoercionLastExpressionUsedAsReturnArgument() {
+    val a = {
+        42
+    }
+
+    a checkType { _<() -> Int>() }
+}
+
+fun noCoercionBlockHasExplicitType() {
+    val <!UNUSED_VARIABLE!>b<!>: () -> Int = {
+        <!TYPE_MISMATCH!>if (true) <!UNUSED_EXPRESSION!>42<!><!>
+    }
+}
+
+fun noCoercionBlockHasExplicitReturn() {
+    val <!UNUSED_VARIABLE!>c<!> = l@{
+        if (true) return@l 42
+
+        <!INVALID_IF_AS_EXPRESSION!>if<!> (true) 239
+    }
+}
+
+fun noCoercionInExpressionBody(): Unit = <!TYPE_MISMATCH!>"hello"<!>