Expose llvm groupdata size as a shadergroup attribute

[chellmuth]

Signed-off-by: Chris Hellmuth chellmuth@gmail.com

Description

Adds the llvm_groupdata_size attribute so the renderer can inspect a shader group's GroupData struct size. On GPU this lets us reserve memory more efficiently.

Tests

No changes.

Checklist:

• [X] I have read the contribution guidelines.
• [X] I have previously submitted a Contributor License Agreement.
• [X] I have updated the documentation, if applicable.
• [ ] I have ensured that the change is tested somewhere in the testsuite (adding new test cases if necessary).
• [X] My code follows the prevailing code style of this project.