[clang] Implement constexpr DesignatedInitUpdateExpr. (#196427)

DesignatedInitUpdateExpr exists to handle some obscure edge cases in C,
where the usual InitListExpr canonicalization can't be performed.
Previously, we didn't need constant evaluation for this, but C23
constexpr means we need to evaluate this before codegen.

Implementation is mostly straightforward: just need to evaluate the two
subexpressions, in order, and skip any NoInitExprs.

Along the way, I ran into an issue with the way we manage array APValues
for non-bytecode constant evaluation; fix reallocation to work
correctly.

Fixes #193373.  Fixes #196450.
6 files changed
tree: 20a2a72384bb3d498bdd7b7ae48df862fd2cfcec
  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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