[TableGen] Ensure that __lsan_is_turned_off isn't removed by DCE in llvm-tblgen

Summary:
Since asan is linked dynamically on Darwin, the weak interface symbol
is removed by -Wl,-dead_strip.

Reviewers: kcc, compnerd, aaron.ballman

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D37636

llvm-svn: 312914