[clang] [ConstEval] Fix crash when comparing strings past the end (PR #137078)

[LLVM Continuous Integration via cfe-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `openmp-offload-amdgpu-runtime-2` running on `rocm-worker-hw-02` while building `clang` at step 7 "Add check check-clang".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/10/builds/4024

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

```
Step 7 (Add check check-clang) failure: test (failure)
******************** TEST 'Clang :: AST/ByteCode/cxx20.cpp' FAILED ********************
Exit Code: 1

Command Output (stderr):
--
/home/botworker/builds/openmp-offload-amdgpu-runtime-2/llvm.build/bin/clang -cc1 -internal-isystem /home/botworker/builds/openmp-offload-amdgpu-runtime-2/llvm.build/lib/clang/21/include -nostdsysteminc -fcxx-exceptions -fexperimental-new-constant-interpreter -std=c++20 -verify=both,expected -fcxx-exceptions /home/botworker/builds/openmp-offload-amdgpu-runtime-2/llvm.src/clang/test/AST/ByteCode/cxx20.cpp -DNEW_INTERP # RUN: at line 1
+ /home/botworker/builds/openmp-offload-amdgpu-runtime-2/llvm.build/bin/clang -cc1 -internal-isystem /home/botworker/builds/openmp-offload-amdgpu-runtime-2/llvm.build/lib/clang/21/include -nostdsysteminc -fcxx-exceptions -fexperimental-new-constant-interpreter -std=c++20 -verify=both,expected -fcxx-exceptions /home/botworker/builds/openmp-offload-amdgpu-runtime-2/llvm.src/clang/test/AST/ByteCode/cxx20.cpp -DNEW_INTERP
error: diagnostics with 'note' severity expected but not seen: 
  File /home/botworker/builds/openmp-offload-amdgpu-runtime-2/llvm.src/clang/test/AST/ByteCode/cxx20.cpp Line 127 'expected-note': comparison against pointer '&"test1"[6] + 1' that points past the end of a complete object has unspecified value
error: diagnostics with 'note' severity seen but not expected: 
  File /home/botworker/builds/openmp-offload-amdgpu-runtime-2/llvm.src/clang/test/AST/ByteCode/cxx20.cpp Line 127: comparison against pointer '&"test1"[6]' that points past the end of a complete object has unspecified value
2 errors generated.

--

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


```

</details>

https://github.com/llvm/llvm-project/pull/137078