[clang] [clang][Sema] Shard out SemaTests into their own binary (PR #155548)

[Reid Kleckner via cfe-commits]

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

PTAL, I edited the comment. The fact that the number of tests is small doesn't really relate to the costs of having a distinct test target, it has more to do with the size of the dependency graph.

https://github.com/llvm/llvm-project/pull/155548