[clang-tools-extra] [clang-tidy] Enable plugin tests with LLVM_INSTALL_TOOLCHAIN_ONLY (PR #90370)

[Igor Kudrin via cfe-commits]

igorkudrin wrote:

> I think out of tree builds of clang-tidy (back in the svn days, when people did partial checkouts of individual projects) probably needed this to be able to correctly correctly find the right headers. Maybe standalone builds aren't permitted anymore? I think I mentioned this a bit in earlier revisions of that PR https://reviews.llvm.org/D111100#3284178

I don't have much to say about standalone builds. I hope to hear about them from the people who mentioned them in [D111100](https://reviews.llvm.org/D111100). But as far as I can see, `clang-tidy-headers` is an empty target, with no dependencies or actions; at least it looks like that when building with `ninja` on Linux or Windows. It seems unlikely that this is different for standalone builds. Removing the dependency on an empty target should be safe.

https://github.com/llvm/llvm-project/pull/90370