[Sanitizers] Add *SAN_OPTIONS to possibly_dangerous_env_vars

Summary:
This way, if they're set when running ninja check-ubsan (or another
sanitizer), they get cleared before we start invoking the programs.

Reviewers: samsonov, kcc

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D10229

llvm-svn: 238991