Check correct async SIL emission

This checks that we generate the correct SIL for async top-level code.
@main sets things up and sets up the task containing `@async_Main`,
which is the actual top-level code. It also verifies that we are
correctly constructing the call to `exit` at the end of `@async_Main`,
ensuring that the program actually terminates.

Author: etcwilde

test/SILGen/toplevel.swift

@@ -1,4 +1,5 @@
-// RUN: %target-swift-emit-silgen -Xllvm -sil-full-demangle %s | %FileCheck %s
+// RUN: %target-swift-emit-silgen -Xllvm -sil-full-demangle %s | %FileCheck %s --check-prefixes='CHECK,SYNC-CHECK'
+// RUN: %target-swift-emit-silgen -Xllvm -sil-full-demangle -enable-experimental-async-top-level %s | %FileCheck %s --check-prefixes='CHECK,ASYNC-CHECK'
 
 func markUsed<T>(_ t: T) {}
 
@@ -10,6 +11,15 @@ func trap() -> Never {
 // CHECK-LABEL: sil [ossa] @main
 // CHECK: bb0({{%.*}} : $Int32, {{%.*}} : $UnsafeMutablePointer<Optional<UnsafeMutablePointer<Int8>>>):
 
+// async setup call to @async_Main (this is in @main)
+// ASYNC-CHECK: [[ASYNC_MAIN:%[0-9]+]] = function_ref @async_Main : $@convention(thin) @async () -> ()
+// ASYNC-CHECK: [[THUNK:%[0-9]+]] = function_ref @$sIetH_yts5Error_pIegHrzo_TR
+
+// Actual body of async top-level
+// ASYNC-CHECK-LABEL: sil hidden [ossa] @async_Main
+// ASYNC-CHECK: bb0:
+
+
 // -- initialize x
 // CHECK: alloc_global @$s8toplevel1xSiv
 // CHECK: [[X:%[0-9]+]] = global_addr @$s8toplevel1xSivp : $*Int
@@ -117,10 +127,11 @@ defer {
 
 
 // CHECK: [[RET:%[0-9]+]] = struct $Int32
-// CHECK: return [[RET]]
-
-
+// SYNC-CHECK: return [[RET]]
 
+// ASYNC-CHECK: [[EXITFUNC:%[0-9]+]] = function_ref @exit
+// ASYNC-CHECK: apply [[EXITFUNC]]([[RET]]) : $@convention(c) (Int32) -> Never
+// ASYNC-CHECK: unreachable
 
 // CHECK-LABEL: sil hidden [ossa] @$s8toplevel7print_xyyF