[asan] Add __asan_set_shadow_*

Summary:
We are poisoning small allocas using store instruction from instrumented code.
For larger allocas we'd like to insert function calls instead of multiple stores.

PR27453

Reviewers: kcc, eugenis

Subscribers: llvm-commits, kubabrecka

Differential Revision: https://reviews.llvm.org/D23616

llvm-svn: 279019