[clang][Sema] Fix a CTAD regression after 42239d2 (llvm#86914)

The most recent declaration of a template as a friend can introduce a
different template parameter depth compared to what we anticipate from a
CTAD guide.

Fixes llvm#86769

clang/docs/ReleaseNotes.rst

@@ -1174,6 +1174,10 @@ Bug Fixes to C++ Support
 - Fix a crash when an unresolved overload set is encountered on the RHS of a ``.*`` operator.
   (`#53815 <https://github.com/llvm/llvm-project/issues/53815>`_)
 
+- Fixed a regression in CTAD that a friend declaration that befriends itself may cause
+  incorrect constraint substitution.
+  (`#86769 <https://github.com/llvm/llvm-project/issues/86769>`_)
+
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^
 - Clang now properly preserves ``FoundDecls`` within a ``ConceptReference``. (#GH82628)