tsan: align default value of detect_deadlocks flag with actual behavior

I tricked myself into thinking that deadlock detection is off by default in TSan by looking at the default value of the detect_deadlocks flag and outdated docs. (Created a pull request to update docs.)
I even managed to confuse others: https://groups.google.com/forum/#!topic/thread-sanitizer/xYvnAYwtoDk

However, the default value is overwritten in code (TSan_flags.cc:InitializeFlags). The TSan/deadlock tests also rely on this

This changes aligns the default value of the flag with the actual default behavior.

Author: yln (Julian Lettner)
Reviewed in: https://reviews.llvm.org/D55846

llvm-svn: 349609