[clang][slh] Add test for SLH feature checking macro

Summary:
I forgot to include a test in this commit:
https://reviews.llvm.org/rG379e68a763097bed55556c6dc7453e4b732e3d68

Here's the test. It passes after that commit and fails before that commit.

Reviewed By: mattdr

Differential Revision: https://reviews.llvm.org/D79894