[clang] [clang][perf-training] Fix training when path to cmake has spaces in it (PR #162919)

[Petr Hosek via cfe-commits]

https://github.com/petrhosek approved this pull request.

I agree with @boomanaiden154, I'm surprised that this didn't show up elsewhere and we'll need to audit other `lit.site.cfg.in` files, but I'm fine merging this change to unblock you.

https://github.com/llvm/llvm-project/pull/162919