[cmake] Fix typo in test/asan/CMakeLists.txt

Summary:
[cmake] Fix typo in test/asan/CMakeLists.txt

Should be variable name instead of variable reference.

Reviewers: samsonov, rnk, smeenai, beanz

Reviewed By: smeenai

Subscribers: kubamracek, llvm-commits, mgorny

Differential Revision: https://reviews.llvm.org/D41214

llvm-svn: 321760