[APIJSON] Impromve tests to it doesn't rely on SDK content

Remove the dependency of APIJSON tests on SDK to improve stability and
speed.

rdar://89448415

Author: cachemeifyoucan

test/APIJSON/apigen.swift

@@ -1,6 +1,6 @@
 // REQUIRES: objc_interop, OS=macosx
 // RUN: %empty-directory(%t)
-// RUN: %target-swift-frontend %s -typecheck -emit-module-interface-path %t/MyModule.swiftinterface -enable-library-evolution -module-name MyModule -swift-version 5 
+// RUN: %target-swift-frontend(mock-sdk: %clang-importer-sdk-nosource -I %t) %s -typecheck -emit-module-interface-path %t/MyModule.swiftinterface -enable-library-evolution -module-name MyModule -swift-version 5 
 // RUN: %target-swift-api-extract -o - -pretty-print %t/MyModule.swiftinterface -module-name MyModule -module-cache-path %t | %FileCheck %s
 
 import Foundation

test/APIJSON/non-objc-class.swift

@@ -1,6 +1,6 @@
 // REQUIRES: objc_interop, OS=macosx
 // RUN: %empty-directory(%t)
-// RUN: %target-swift-frontend %s -typecheck -emit-module-interface-path %t/MyModule.swiftinterface -enable-library-evolution -module-name MyModule -swift-version 5
+// RUN: %target-swift-frontend(mock-sdk: %clang-importer-sdk-nosource -I %t) %s -typecheck -emit-module-interface-path %t/MyModule.swiftinterface -enable-library-evolution -module-name MyModule -swift-version 5
 // RUN: %target-swift-api-extract -o - -pretty-print %t/MyModule.swiftinterface -module-name MyModule -module-cache-path %t | %FileCheck %s
 
 import Foundation

test/APIJSON/spi.swift

@@ -1,9 +1,9 @@
 // REQUIRES: objc_interop, OS=macosx
 // RUN: %empty-directory(%t)
-// RUN: %target-swift-frontend %s -typecheck -emit-module-interface-path %t/MyModule.swiftinterface -enable-library-evolution -module-name MyModule -swift-version 5
+// RUN: %target-swift-frontend(mock-sdk: %clang-importer-sdk-nosource -I %t) %s -typecheck -emit-module-interface-path %t/MyModule.swiftinterface -enable-library-evolution -module-name MyModule -swift-version 5
 // RUN: %target-swift-api-extract -o - -pretty-print %t/MyModule.swiftinterface -module-name MyModule -module-cache-path %t | %FileCheck %s
 
-// RUN: %target-swift-frontend %s -emit-module -emit-module-path %t/MyModule.swiftmodule -enable-library-evolution -module-name MyModule -swift-version 5
+// RUN: %target-swift-frontend(mock-sdk: %clang-importer-sdk-nosource -I %t) %s -emit-module -emit-module-path %t/MyModule.swiftmodule -enable-library-evolution -module-name MyModule -swift-version 5
 // RUN: %target-swift-api-extract -o - -pretty-print %t/MyModule.swiftmodule -module-name MyModule -module-cache-path %t | %FileCheck %s --check-prefix=CHECK-SPI
 
 import Foundation