[Clang] prevent an assertion failure caused by C++ constant expression checks in C23 floating conversions (#174113)

Fixes #173847

---

This patch addresses an assertion failure during compilation of C23 code
involving floating-point conversions.

As part of the C23 constexpr support introduced in PR #73099, Clang
began reusing parts of the C++ constant evaluation and narrowing logic.
In C23 mode, a failed constant evaluation caused the condition to
proceed to C++ constant-expression checks, resulting in an assertion
failure.

This change evaluates constants using `EvaluateAsRValue` in C23 mode and
restricts C++ constant-expression checks to C++ mode.
3 files changed
tree: 75be000069a77def2cec609036b0558caa0a22e9
  1. .ci/
  2. .github/
  3. bolt/
  4. clang/
  5. clang-tools-extra/
  6. cmake/
  7. compiler-rt/
  8. cross-project-tests/
  9. flang/
  10. flang-rt/
  11. libc/
  12. libclc/
  13. libcxx/
  14. libcxxabi/
  15. libsycl/
  16. libunwind/
  17. lld/
  18. lldb/
  19. llvm/
  20. llvm-libgcc/
  21. mlir/
  22. offload/
  23. openmp/
  24. orc-rt/
  25. polly/
  26. runtimes/
  27. third-party/
  28. utils/
  29. .clang-format
  30. .clang-format-ignore
  31. .clang-tidy
  32. .git-blame-ignore-revs
  33. .gitattributes
  34. .gitignore
  35. .mailmap
  36. CODE_OF_CONDUCT.md
  37. CONTRIBUTING.md
  38. LICENSE.TXT
  39. pyproject.toml
  40. README.md
  41. SECURITY.md
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