Do not coerce function literals to suspend.

The design is to use `suspend fun` instead of coercion, just as suspend
lambdas.
However, this syntax is not supported in the parser. But this is not a
problem, since the coercion lead to internal compiler error.
As a workaround everybody uses suspend lambdas.
 #KT-24860: Fixed

compiler/testData/diagnostics/tests/inference/coerceFunctionLiteralToSuspend.kt

@@ -0,0 +1,29 @@
+// !DIAGNOSTICS: -UNUSED_PARAMETER
+
+fun fail1(c: suspend () -> Unit) {}
+
+fun fail2(c: () -> Unit) {}
+
+fun success1(c: suspend () -> Unit) {}
+
+fun test1() {
+    fail1(<!TYPE_MISMATCH!>fun () {}<!>)
+    fun fail2(c: suspend () -> Unit) {}
+    fail2(<!TYPE_MISMATCH!>fun () {}<!>)
+    fun success1(c: () -> Unit) {}
+    success1(fun() {})
+}
+
+suspend fun fail3(c: suspend () -> Unit) {}
+
+suspend fun fail4(c: () -> Unit) {}
+
+suspend fun success2(c: suspend () -> Unit) {}
+
+suspend fun test2() {
+    fail3(<!TYPE_MISMATCH!>fun () {}<!>)
+    fun fail4(c: suspend () -> Unit) {}
+    fail4(<!TYPE_MISMATCH!>fun () {}<!>)
+    fun success2(c: () -> Unit) {}
+    success2(fun() {})
+}