[Test] Don't test _swift_willThrow on older runtimes.

test/stdlib/Error.swift

@@ -193,25 +193,27 @@ ErrorTests.test("test dealloc empty error box") {
 
 var errors: [Error] = []
 ErrorTests.test("willThrow") {
-  // Error isn't allowed in a @convention(c) function when ObjC interop is
-  // not available, so pass it through an OpaquePointer.
-  typealias WillThrow = @convention(c) (OpaquePointer) -> Void
-  let willThrow = pointerToSwiftCoreSymbol(name: "_swift_willThrow")!
-  let callback: WillThrow = {
-    errors.append(unsafeBitCast($0, to: Error.self))
-  }
-  willThrow.storeBytes(of: callback, as: WillThrow.self)
-  expectTrue(errors.isEmpty)
-  do {
-    throw UnsignedError.negativeOne
-  } catch {}
-  expectEqual(UnsignedError.self, type(of: errors.last!))
+  if #available(macOS 9999, iOS 9999, watchOS 9999, tvOS 9999, *) {
+    // Error isn't allowed in a @convention(c) function when ObjC interop is
+    // not available, so pass it through an OpaquePointer.
+    typealias WillThrow = @convention(c) (OpaquePointer) -> Void
+    let willThrow = pointerToSwiftCoreSymbol(name: "_swift_willThrow")!
+    let callback: WillThrow = {
+      errors.append(unsafeBitCast($0, to: Error.self))
+    }
+    willThrow.storeBytes(of: callback, as: WillThrow.self)
+    expectTrue(errors.isEmpty)
+    do {
+      throw UnsignedError.negativeOne
+    } catch {}
+    expectEqual(UnsignedError.self, type(of: errors.last!))
 
-  do {
-    throw SillyError.JazzHands
-  } catch {}
-  expectEqual(2, errors.count)
-  expectEqual(SillyError.self, type(of: errors.last!))
+    do {
+      throw SillyError.JazzHands
+    } catch {}
+    expectEqual(2, errors.count)
+    expectEqual(SillyError.self, type(of: errors.last!))
+  }
 }
 
 runAllTests()