[clang] [clang][cmake] Move perf-training out of CLANG_INCLUDE_TESTS (PR #192163)

[Petr Hosek via cfe-commits]

petrhosek wrote:

> Ping @petrhosek - the commit message has been rewritten per your feedback (Assisted-by: Claude trailer added). Could you please take another look when you get a chance? Thanks!

The change LGTM but note that GitHub doesn't use the commit message, rather it uses the PR description as the merged commit message, so you should update the description prior to merging.

https://github.com/llvm/llvm-project/pull/192163