[llvm] [CI] Switch to a single runtimes build (PR #131913)

[Tom Stellard via llvm-commits]

tstellar wrote:

> 
> This was discussed during Clang WG meeting yesterday. There were no objections to cut C++03 configuration, but C++26 and Clang Modules configurations were deemed important. Can we start with removing C++03?

This seems like a reasonable first step to me.  If we are still having queue delays maybe we need to bring the discussion to Discourse and have a wider discussion about testing priorities.  We have similar issues with long-queuing delays for the free macOS runners, but we don't have a good process to determine which workloads to prioritize.


https://github.com/llvm/llvm-project/pull/131913