Adds a CAS logging interface to track mutations of the CAS and action
cache database files. This is designed to aid in the debugging of
data corruption and other issues with the low-level on-disk
representation.
To enable logging, set the environment variable LLVM_CAS_LOG to 1 or 2.
Level 2 includes logging of all allocations inside the index, action
cache, and datapool, which increases the overhead. The log file is
placed inside the top-level directory of the unified cache. For now it
is a human-readable textual format with one line per entry.
