[FIR] fix internal error on suspend conversion with not-computed return type

^KT-62836 Fixed

compiler/testData/diagnostics/tests/suspendConversion/implicitInvoke.kt

@@ -0,0 +1,8 @@
+// FIR_IDENTICAL
+fun box() = useSuspendFunInt(Test())
+
+fun useSuspendFunInt(fn: suspend () -> String): String = ""
+
+open class Test : () -> String {
+    override fun invoke() = "OK"
+}