[asan] add primitives that allow coroutine implementations

This patch adds the __sanitizer_start_switch_fiber and
__sanitizer_finish_switch_fiber methods inspired from what can be found here
https://github.com/facebook/folly/commit/2ea64dd24946cbc9f3f4ac3f6c6b98a486c56e73 .

These methods are needed when the compiled software needs to implement
coroutines, fibers or the like. Without a way to annotate them, when the program
jumps to a stack that is not the thread stack, __asan_handle_no_return shows a
warning about that, and the fake stack mechanism may free fake frames that are
still in use.

Author: blastrock (Philippe Daouadi)
Reviewed in http://reviews.llvm.org/D20913

llvm-svn: 273260