Clear empty unevac regions in ClearFromSpace

The motivation is to release RAM for empty unevac regions at the end
of the current GC instead of next GC.

Results on RitzPerf:
This optimization accounts for 2.33GB out of 53GB of freed bytes ~4%.
This also means that the average heap size is probably 4% smaller,
though it may not translate to RAM savings for actual apps.

Bug: 35800768
Bug: 12687968

Test: test-art-host
Change-Id: I38fa8dcda1aa6b439060ff170941b3a53022846f