Add a release note

Author: zyn0217

clang/docs/ReleaseNotes.rst

@@ -264,6 +264,8 @@ Bug Fixes to C++ Support
 - Clang is now better at keeping track of friend function template instance contexts. (#GH55509)
 - Clang now prints the correct instantiation context for diagnostics suppressed
   by template argument deduction.
+- Clang is now better at instantiating the function definition after its use inside
+  of a constexpr lambda. (#GH125747)
 - The initialization kind of elements of structured bindings
   direct-list-initialized from an array is corrected to direct-initialization.
 - Clang no longer crashes when a coroutine is declared ``[[noreturn]]``. (#GH127327)