[cmake] Add llvm-locstats to LLVM_TEST_DEPENDS

Summary:
Make sure that llvm-locstats is updated before running lit tests even
when it's not an explicit target.

Reviewers: djtodoro, krisb, spatel

Reviewed By: djtodoro

Subscribers: mgorny, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D71611