[asan] Remove runtime flag detect_stack_use_after_scope

Summary:
We are going to use store instructions to poison some allocas.
Runtime flag will require branching in instrumented code on every lifetime
intrinsic. We'd like to avoid that.

Reviewers: eugenis

Subscribers: llvm-commits, kubabrecka

Differential Revision: https://reviews.llvm.org/D23967

llvm-svn: 279981