[asan] Remove allow_user_segv_handler on Windows.

Summary:
This flags is not covered by tests on Windows and looks like it's implemented
incorrectly. Switching its default breaks some tests.

Taking into account that related handle_segv flag is not supported on Windows
it's safer to remove it until we commit to support it.

Reviewers: eugenis, zturner, rnk

Subscribers: kubamracek, llvm-commits

Differential Revision: https://reviews.llvm.org/D33471

llvm-svn: 303728