serialization: obfuscate the serialized search paths

We noticed some Swift clients rely on the serialized search paths in the module to
find dependencies and droping these paths altogether can lead to build failures like
rdar://85840921.

This change teaches the serialization to obfuscate the search paths and the deserialization
to recover them. This allows clients to keep accessing these paths without exposing
them when shipping the module to other users.

include/swift/AST/Module.h

@@ -833,6 +833,8 @@ public:
   void collectBasicSourceFileInfo(
       llvm::function_ref<void(const BasicSourceFileInfo &)> callback) const;
 
+  void collectSerializedSearchPath(
+      llvm::function_ref<void(StringRef)> callback) const;
   /// Retrieve a fingerprint value that summarizes the contents of this module.
   ///
   /// This interface hash a of a module is guaranteed to change if the interface