Use store operation to poison allocas for lifetime analysis.

Summary:
Calling __asan_poison_stack_memory and __asan_unpoison_stack_memory for small
variables is too expensive.

Code is disabled by default and can be enabled by -asan-experimental-poisoning.

PR27453

Reviewers: eugenis

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D23947

llvm-svn: 279984