[Test] Do not load string processing when recovery is disabled

This test specifically disables serialization recovery, but that can
cause issues when loading the experimental string processing library. We
don't use it in the test, so just disable it for now.

Author: bnbarham

test/Index/skip-loaded-internal.swift

@@ -6,7 +6,7 @@
 // RUN: %target-swift-frontend -emit-module -emit-module-source-info -module-name lib2 -o %t/Frameworks/lib2.framework/Modules/lib2.swiftmodule/%module-target-triple.swiftmodule %t/lib2.swift
 // RUN: %target-swift-frontend -emit-module -emit-module-source-info -module-name lib -o %t/Frameworks/lib.framework/Modules/lib.swiftmodule/%module-target-triple.swiftmodule %t/lib.swift -Fsystem %t/Frameworks
 
-// RUN: %target-swift-frontend -typecheck -index-system-modules -index-ignore-stdlib -index-store-path %t/idx -Fsystem %t/Frameworks %t/main.swift -disable-deserialization-recovery
+// RUN: %target-swift-frontend -typecheck -index-system-modules -index-ignore-stdlib -index-store-path %t/idx -Fsystem %t/Frameworks %t/main.swift -disable-deserialization-recovery -disable-experimental-string-processing
 
 //--- main.swift
 import lib