[asan] Add new hidden compile-time flag asan-instrument-allocas to sanitize variable-sized dynamic allocas. Patch by Max Ostapenko.

Reviewed at http://reviews.llvm.org/D6055


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222519 91177308-0d34-0410-b5e6-96231b3b80d8