Add opt-in option to turn on ThinLTO caching

Allow developers to enable ThinLTO caching on their local build by
setting USE_THINLTO_CACHE environment variable.

This significantly speeds up incremental ThinLTO builds by caching
intermediate results. An incremental ART build (by `touch
compiler/compiler.cc`) reduced from 01:32 to 00:45.

We put the cache under out/soong/thinlto-cache as the cache can be
valid across different targets, and allows us to set a global cache
pruning policy.

The current cache policy is sufficient for current list of
ThinLTO-enabled projects, we can tune it up/down in the future.

Test: USE_THINLTO_CACHE=true m
Bug: 62839002
Change-Id: I4838fe833a1d8fbc73d743da6d19e3698dc89600