Add proper availability annotations for test

Author: DougGregor

test/Macros/macro_expand_body.swift

@@ -21,6 +21,7 @@ extension String: ConjureRemoteValue {
   static func conjureValue() -> String { "" }
 }
 
+@available(SwiftStdlib 5.1, *)
 func remoteCall<Result: ConjureRemoteValue>(function: String, arguments: [String: Any]) async throws -> Result {
   let printedArgs = arguments.keys.sorted().map { key in 
     "\(key): \(arguments[key]!)"
@@ -29,16 +30,19 @@ func remoteCall<Result: ConjureRemoteValue>(function: String, arguments: [String
   return Result.conjureValue()
 }
 
+@available(SwiftStdlib 5.1, *)
 @Remote
 func f(a: Int, b: String) async throws -> String
 
 
+@available(SwiftStdlib 5.1, *)
 @Remote
 func g(a: Int, b: String) async throws -> String {
   doesNotTypeCheck()
 }
 
 #if compiler(>=5.11) && TEST_DIAGNOSTICS
+@available(SwiftStdlib 5.1, *)
 @Remote
 func h(a: Int, b: String) async throws -> String {
   does not
@@ -47,8 +51,10 @@ func h(a: Int, b: String) async throws -> String {
 }
 #endif
 
-// CHECK: Remote call f(a: 5, b: Hello)
-print(try await f(a: 5, b: "Hello"))
+if #available(SwiftStdlib 5.1, *) {
+  // CHECK: Remote call f(a: 5, b: Hello)
+  print(try await f(a: 5, b: "Hello"))
 
-// CHECK: Remote call g(a: 5, b: World)
-print(try await g(a: 5, b: "World"))
+  // CHECK: Remote call g(a: 5, b: World)
+  print(try await g(a: 5, b: "World"))
+}