[6.2] use RespectOriginallyDefinedIn when mangling extension contexts (#82657)

- **Explanation**: USR mangling can include an extension context infix
(`AAE`) when an extended type uses `@_originallyDefinedIn` on platforms
other than the active one. This adds a check for the
`RespectOriginallyDefinedIn` flag when checking extension decls against
their extended type.
- **Scope**: Changes USR mangling in these situations so that USRs are
the same for the same code regardless of platform.
- **Issues**: rdar://152598492
- **Original PRs**: https://github.com/swiftlang/swift/pull/82348
- **Risk**: Low. The change is limited to situations where the name
mangler is already disrespecting the alternate module name, and only
additionally turns on that flag for any USR mangling.
- **Testing**: Automated tests
- **Reviewers**: @edymtt @augusto2112

include/swift/AST/Decl.h

@@ -2135,7 +2135,10 @@ public:
 
   /// Determine whether this extension context is in the same defining module as
   /// the original nominal type context.
-  bool isInSameDefiningModule() const;
+  ///
+  /// \param RespectOriginallyDefinedIn Whether to respect
+  /// \c @_originallyDefinedIn attributes or the actual location of the decls.
+  bool isInSameDefiningModule(bool RespectOriginallyDefinedIn = true) const;
 
   /// Determine whether this extension is equivalent to one that requires at
   /// at least some constraints to be written in the source.