Add new ASAN_OPTION: sleep_after_init.

Summary: As mentioned in https://github.com/google/sanitizers/issues/834, suggested option can be handy for debugging.

Reviewers: kcc

Reviewed By: kcc

Subscribers: llvm-commits, kubamracek

Differential Revision: https://reviews.llvm.org/D35409

llvm-svn: 309854