[asan] Runtime support for asan-instrument-allocas which enables instrumentation of variable-sized dynamic allocas. Patch by Max Ostapenko.

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


git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@222520 91177308-0d34-0410-b5e6-96231b3b80d8