[clang][deps] Use lock_guard instead of unique_lock

This patch changes uses of `std::unique_lock` to `std::lock_guard`.

The `std::unique_lock` template provides some advanced capabilities (deferred locking, time-constrained locking attempts, etc.) we don't use in the caching filesystem. Plain `std::lock_guard` will do here.

Reviewed By: dexonsmith

Differential Revision: https://reviews.llvm.org/D115332