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/Serialization/SerializationOptions.h

@@ -45,6 +45,10 @@ namespace swift {
     /// Path prefixes that should be rewritten in debug info.
     PathRemapper DebuggingOptionsPrefixMap;
 
+    /// Obfuscate the serialized paths so we don't have the actual paths encoded
+    /// in the .swiftmodule file.
+    PathObfuscator PathObfuscator;
+
     /// Describes a single-file dependency for this module, along with the
     /// appropriate strategy for how to verify if it's up-to-date.
     class FileDependency {