[Test] Used test header rather than SDK.

Stopped validation-test/compiler_crashers_2_fixed/rdar79383990.swift
from trying to call -[NSBackgroundActivityScheduler scheduleWithBlock:]
async--that method is now annotated NS_SWIFT_DISABLE_ASYNC.

(cherry picked from commit 4a8a647)
(cherry picked from commit bdbf76a)

Author: nate-chandler

validation-test/compiler_crashers_2_fixed/Inputs/rdar79383990.h

@@ -0,0 +1,12 @@
+@import Foundation;
+
+typedef NS_ENUM(NSInteger, BackgroundActivityResult) {
+    BackgroundActivityResultFinished = 1,
+    BackgroundActivityResultDeferred = 2,
+};
+
+typedef void (^BackgroundActivityCompletionHandler)(BackgroundActivityResult result);
+
+@interface BackgroundActivityScheduler : NSObject
+- (void)scheduleWithBlock:(void (^)(BackgroundActivityCompletionHandler completionHandler))block;
+@end