[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 `clang-x86_64-debian-fast` running on `gribozavr4` while building `clang` at step 6 "test-build-unified-tree-check-all".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/56/builds/24173

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

```
Step 6 (test-build-unified-tree-check-all) failure: test (failure)
******************** TEST 'Clang :: AST/ByteCode/cxx20.cpp' FAILED ********************
Exit Code: 1

Command Output (stderr):
--
/b/1/clang-x86_64-debian-fast/llvm.obj/bin/clang -cc1 -internal-isystem /b/1/clang-x86_64-debian-fast/llvm.obj/lib/clang/21/include -nostdsysteminc -fcxx-exceptions -fexperimental-new-constant-interpreter -std=c++20 -verify=both,expected -fcxx-exceptions /b/1/clang-x86_64-debian-fast/llvm.src/clang/test/AST/ByteCode/cxx20.cpp -DNEW_INTERP # RUN: at line 1
+ /b/1/clang-x86_64-debian-fast/llvm.obj/bin/clang -cc1 -internal-isystem /b/1/clang-x86_64-debian-fast/llvm.obj/lib/clang/21/include -nostdsysteminc -fcxx-exceptions -fexperimental-new-constant-interpreter -std=c++20 -verify=both,expected -fcxx-exceptions /b/1/clang-x86_64-debian-fast/llvm.src/clang/test/AST/ByteCode/cxx20.cpp -DNEW_INTERP
error: diagnostics with 'note' severity expected but not seen: 
  File /b/1/clang-x86_64-debian-fast/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 /b/1/clang-x86_64-debian-fast/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