[clangd] Avoid combinatorial explosion in CodeCompleteTests.

Summary:
This test dominates our unit test runtime, and the change speeds it up by 10x.
We lose coverage of some combinations of flags, but I'm not sure that's finding
many bugs.

3300 -> 300ms on my machine (3800 -> 800ms for the whole of CompletionTest).

Reviewers: ilya-biryukov

Subscribers: klimek, cfe-commits

Differential Revision: https://reviews.llvm.org/D42063

llvm-svn: 322547