[Clang] Fixes to immediate-escalating functions (llvm#82281)

* Consider that immediate escalating function can appear at global
scope, fixing a crash

* Lambda conversion to function pointer was sometimes not performed in
an immediate function context when it should be.

Fixes llvm#82258

(cherry picked from commit baf6bd3)

Author: cor3ntin

clang/docs/ReleaseNotes.rst

@@ -1167,6 +1167,11 @@ Bug Fixes to C++ Support
 - Fix incorrect code generation caused by the object argument of ``static operator()`` and ``static operator[]`` calls not being evaluated.
   Fixes (`#67976 <https://github.com/llvm/llvm-project/issues/67976>`_)
 
+- Fix crash when using an immediate-escalated function at global scope.
+  (`#82258 <https://github.com/llvm/llvm-project/issues/82258>`_)
+- Correctly immediate-escalate lambda conversion functions.
+  (`#82258 <https://github.com/llvm/llvm-project/issues/82258>`_)
+
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^
 - Clang now properly preserves ``FoundDecls`` within a ``ConceptReference``. (#GH82628)