Prevent DCE on __lsan_is_turned_off and re-enable test case

Summary:
-dead_strip in ld64 strips weak interface symbols, which I believe
is most likely the cause of this test failure. Re-enable after marking the interface
function as used.

Reviewers: alekseyshl, kubamracek, kcc

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D37635

llvm-svn: 312824