[HLSL][Matrix] Add APValue and ConstExpr evaluator support for matrices (#178762)

Fixes #168935

This PR adds basic support for matrix APValues and a ConstExpr evaluator
for matrices.
- ConstExpr evaluation changes:
  - Matrix initializer list
  - Matrix HLSL elementwise cast
  - Matrix HLSL aggregate splat
  - Vector HLSL matrix truncation
  - Int HLSL matrix truncation
  - Float HLSL matrix truncation
- Matrix APValue:
  - AST dumper and serialization
  - Value flattening

Note that APValue matrices hold its elements in row-major order
irrespective of the `-fmatrix-memory-layout` flag.
The `-fmatrix-memory-layout` is for codegen, not semantics, so the
decision of which memory layout to use for the matrix APValue can be
independent of the memory layout for codegen.

There are also a number of places expecting switch case coverage over
all APValues but which do not currently support matrix APValues. I have
added placeholder llvm_unreachables to these places for the matrix
APValue, as these places can not currently be exercised in clang tests
(AFAIK).

Assisted-by: claude-opus-4.5
15 files changed
tree: 6c11fbbf43bff88f2a9ff6ffc844c68a3deb182f
  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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