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

[Jameson Nash via cfe-commits]

vtjnash 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

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