Use -Og clang option for debug builds (instead of -O2).

This is very similar to O1 (many optimizations still enabled),
with the explicit goal of making the code somewhat debuggable.

For example, this reduces the amount of "optimized away" variables,
while having minimal impact on the execution time of tests (<5%).

Test: time ./art/test.py -r --host --optimizing --64
Change-Id: Icd155211383520d39c3e82a4e2d4975d9573a36d
1 file changed