[llvm] [clang] Disable runtimes testing in C++03 and Clang modules modes (PR #99303)

[Aaron Ballman via llvm-commits]

AaronBallman wrote:

The changes LGTM, but I'd like to wait to land them until there's been a chance for some discussion on the Discourse post and we've heard from @ldionne.

https://github.com/llvm/llvm-project/pull/99303