Adding expected resolution test case

The test case is the situation that causes problems. Specifically, for
`Config1`, you would expect the `main()` function where
`Configuration == Config1` to be selected, but because we favored
asynchronous main functions, we would select the one from the default
`App` extension, which is bad.

Author: etcwilde

test/Concurrency/where_clause_main_resolution.swift

@@ -0,0 +1,45 @@
+// RUN: %target-swift-frontend -disable-availability-checking -D CONFIG1 -dump-ast -parse-as-library %s | %FileCheck %s --check-prefix=CHECK-CONFIG1
+// RUN: %target-swift-frontend -disable-availability-checking -D CONFIG2 -dump-ast -parse-as-library %s | %FileCheck %s --check-prefix=CHECK-CONFIG2
+// RUN: %target-swift-frontend -disable-availability-checking -D CONFIG3 -dump-ast -parse-as-library %s | %FileCheck %s --check-prefix=CHECK-CONFIG3
+
+// REQUIRES: concurrency
+
+protocol AppConfiguration { }
+
+struct Config1: AppConfiguration {}
+struct Config2: AppConfiguration {}
+struct Config3: AppConfiguration {}
+
+protocol App {
+    associatedtype Configuration: AppConfiguration
+}
+
+extension App where Configuration == Config1 {
+// CHECK-CONFIG1: (func_decl implicit "$main()" interface type='(MainType.Type) -> () -> ()'
+// CHECK-CONFIG1: where_clause_main_resolution.swift:[[# @LINE+1 ]]
+    static func main() { }
+}
+
+extension App where Configuration == Config2 {
+// CHECK-CONFIG2: (func_decl implicit "$main()" interface type='(MainType.Type) -> () async -> ()'
+// CHECK-CONFIG2: where_clause_main_resolution.swift:[[# @LINE+1 ]]
+    static func main() async { }
+}
+
+extension App {
+// CHECK-CONFIG3: (func_decl implicit "$main()" interface type='(MainType.Type) -> () async -> ()'
+// CHECK-CONFIG3: where_clause_main_resolution.swift:[[# @LINE+1 ]]
+    static func main() async { }
+}
+
+@main
+struct MainType : App {
+
+#if CONFIG1
+    typealias Configuration = Config1
+#elseif CONFIG2
+    typealias Configuration = Config2
+#elseif CONFIG3
+    typealias Configuration = Config3
+#endif
+}