[asan] Speed up ASan unit tests by turning off symbolication

ASan unit tests don't depend on the symbolizer and they never test its output, but currently they still run it for every crash. This patch simply disables the symbolizer for unit tests, which provides some speed up. On my system (OS X) that's about 1.4x faster, but this could be potentially much more e.g. when using atos on OS X.

Differential Revision: http://reviews.llvm.org/D11353

llvm-svn: 242677