[clang] [Clang] Implement P2280R4 Using unknown pointers and references in constant expressions (PR #95474)

[LLVM Continuous Integration via cfe-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `llvm-x86_64-debian-dylib` running on `gribozavr4` while building `clang` at step 6 "test-build-unified-tree-check-clang".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/60/builds/17619

<details>
<summary>Here is the relevant piece of the build log for the reference</summary>

```
Step 6 (test-build-unified-tree-check-clang) failure: test (failure)
******************** TEST 'Clang :: AST/ByteCode/cxx2a.cpp' FAILED ********************
Exit Code: 1

Command Output (stderr):
--
RUN: at line 1: /b/1/llvm-x86_64-debian-dylib/build/bin/clang -cc1 -internal-isystem /b/1/llvm-x86_64-debian-dylib/build/lib/clang/20/include -nostdsysteminc -std=c++2a -fsyntax-only -fcxx-exceptions -verify=ref,both /b/1/llvm-x86_64-debian-dylib/llvm-project/clang/test/AST/ByteCode/cxx2a.cpp
+ /b/1/llvm-x86_64-debian-dylib/build/bin/clang -cc1 -internal-isystem /b/1/llvm-x86_64-debian-dylib/build/lib/clang/20/include -nostdsysteminc -std=c++2a -fsyntax-only -fcxx-exceptions -verify=ref,both /b/1/llvm-x86_64-debian-dylib/llvm-project/clang/test/AST/ByteCode/cxx2a.cpp
error: 'both-error' diagnostics expected but not seen: 
  File /b/1/llvm-x86_64-debian-dylib/llvm-project/clang/test/AST/ByteCode/cxx2a.cpp Line 142: constant expression
error: 'both-note' diagnostics expected but not seen: 
  File /b/1/llvm-x86_64-debian-dylib/llvm-project/clang/test/AST/ByteCode/cxx2a.cpp Line 142: typeid applied to object 'extern_b2' whose dynamic type is not constant
2 errors generated.

--

********************


```

</details>

https://github.com/llvm/llvm-project/pull/95474