Partially revert r154390 (http://llvm.org/viewvc/llvm-project?view=rev&revision=154390)

Until we work out the solution for http://code.google.com/p/address-sanitizer/issues/detail?id=65 we'd better not allow
the clients to override AddressSanitizer's signal handler.

The second part of r154390 (removing the sighandler-related tests) is not reverted, because those tests were broken
and didn't test anything.

llvm-svn: 154803