[HLSL][SPIRV] Add vk::constant_id attribute. (#143544)

The vk::constant_id attribute is used to indicate that a global const
variable
represents a specialization constant in SPIR-V. This PR adds this
attribute to clang.

The documentation for the attribute is
[here](https://github.com/microsoft/DirectXShaderCompiler/blob/main/docs/SPIR-V.rst#specialization-constants).

The strategy is to to modify the initializer to get the value of a
specialize constant for a builtin defined in the SPIR-V backend.

Implements https://github.com/llvm/wg-hlsl/pull/287

Fixes https://github.com/llvm/llvm-project/issues/142448

---------

Co-authored-by: Nathan Gauër <github@keenuts.net>
GitOrigin-RevId: acde20b5605f3a3a8da2217e4526fc045e6603ed
15 files changed
tree: 64473c2f734f9e4100fbc93f533fe167883e649b
  1. bindings/
  2. cmake/
  3. docs/
  4. examples/
  5. include/
  6. lib/
  7. runtime/
  8. test/
  9. tools/
  10. unittests/
  11. utils/
  12. www/
  13. .clang-format
  14. .clang-tidy
  15. .gitignore
  16. AreaTeamMembers.txt
  17. CMakeLists.txt
  18. INSTALL.txt
  19. LICENSE.TXT
  20. Maintainers.rst
  21. NOTES.txt
  22. README.md
README.md

C language Family Front-end

Welcome to Clang.

This is a compiler front-end for the C family of languages (C, C++ and Objective-C) which is built as part of the LLVM compiler infrastructure project.

Unlike many other compiler frontends, Clang is useful for a number of things beyond just compiling code: we intend for Clang to be host to a number of different source-level tools. One example of this is the Clang Static Analyzer.

If you're interested in more (including how to build Clang) it is best to read the relevant websites. Here are some pointers: