Fix false USELESS_CAST in function with expression body

Note that current behaviour is made similar to the case with
properties initializers/accessors, which means that more complex
cases are not covered yet (see KT-20801) #KT-20802 fixed.

compiler/testData/diagnostics/tests/cast/AsInExpressionBody.kt

@@ -0,0 +1,12 @@
+fun <T> id(x: T): T = x
+
+fun foo() = 1 as Any
+fun bar() = id(1) as Any
+fun baz() = (1 + 1) as Any
+
+val functionLiteral1 = fun() = 1 as Any
+val functionLiteral2 = fun() = id(1) as Any
+val functionLiteral3 = fun() = (1 + 1) as Any
+
+// TODO: this and more complex cases are not supported yet
+fun baz(b: Boolean) = if (b) 1 <!USELESS_CAST!>as Any<!> else 42 <!USELESS_CAST!>as Any?<!>

compiler/testData/diagnostics/tests/cast/AsInExpressionBody.txt

@@ -0,0 +1,10 @@
+package
+
+public val functionLiteral1: () -> kotlin.Any
+public val functionLiteral2: () -> kotlin.Any
+public val functionLiteral3: () -> kotlin.Any
+public fun bar(): kotlin.Any
+public fun baz(): kotlin.Any
+public fun baz(/*0*/ b: kotlin.Boolean): kotlin.Any?
+public fun foo(): kotlin.Any
+public fun </*0*/ T> id(/*0*/ x: T): T

compiler/testData/diagnostics/tests/cast/nothingAs.kt

@@ -23,9 +23,9 @@ fun test() {
     val f = TODO() as Interface
 }
 
-fun a() = TODO() <!USELESS_CAST!>as Any<!>
-fun b() = TODO() <!USELESS_CAST!>as Any?<!>
-fun c() = TODO() <!USELESS_CAST!>as OpenClass<!>
-fun d() = TODO() <!USELESS_CAST!>as FinalClass<!>
-fun e() = TODO() <!USELESS_CAST!>as AbstractClass<!>
-fun f() = TODO() <!USELESS_CAST!>as Interface<!>
+fun a() = TODO() as Any
+fun b() = TODO() as Any?
+fun c() = TODO() as OpenClass
+fun d() = TODO() as FinalClass
+fun e() = TODO() as AbstractClass
+fun f() = TODO() as Interface