Enable -fsanitize=use-after-return by default under -fsanitize=address

Summary:
We enable ASAN's use-after-return instrumentation at compile-time,
but still keep it disabled at run-time.
This enables the users to flip the flag at run-time using environment variable
ASAN_OPTIONS=detect_stack_use_after_return=1 instead of using a separate build.
If UAR detection is disabled at run-time, this extra compile-time instrumentation
costs very small slowdown. On SPEC 2006 14 tests are not affected at all,
4 tests get ~ 1% slowdown and 453.povray gets 4%.

Reviewers: samsonov

Reviewed By: samsonov

CC: llvm-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D1741