[stdlib] Make StdlibUnittest nullability-agnostic for ObjC runtime functions.

Author: Greg Parker

stdlib/private/StdlibUnittest/StdlibUnittest.swift.gyb

@@ -629,7 +629,7 @@ func _childProcess() {
 
 #if _runtime(_ObjC)
   objc_setUncaughtExceptionHandler {
-    let exception = $0! as AnyObject
+    let exception = ($0 as Optional)! as AnyObject
     var stderr = _Stderr()
     let maybeNSException =
         unsafeBitCast(exception, to: _StdlibUnittestNSException.self)

stdlib/private/StdlibUnittestFoundationExtras/StdlibUnittestFoundationExtras.swift

@@ -27,13 +27,17 @@ public func withOverriddenLocaleCurrentLocale<Result>(
   _ temporaryLocale: NSLocale,
   _ body: () -> Result
 ) -> Result {
-  let oldMethod = class_getClassMethod(
-    NSLocale.self, #selector(getter: NSLocale.current))
-  precondition(oldMethod != nil, "could not find +[Locale currentLocale]")
+  guard let oldMethod = class_getClassMethod(
+    NSLocale.self, #selector(getter: NSLocale.current)) as Optional
+  else {
+    _preconditionFailure("could not find +[Locale currentLocale]")
+  }
 
-  let newMethod = class_getClassMethod(
-    NSLocale.self, #selector(NSLocale._swiftUnittest_currentLocale))
-  precondition(newMethod != nil, "could not find +[Locale _swiftUnittest_currentLocale]")
+  guard let newMethod = class_getClassMethod(
+    NSLocale.self, #selector(NSLocale._swiftUnittest_currentLocale)) as Optional
+  else {
+    _preconditionFailure("could not find +[Locale _swiftUnittest_currentLocale]")
+  }
 
   precondition(_temporaryLocaleCurrentLocale == nil,
     "nested calls to withOverriddenLocaleCurrentLocale are not supported")