[test] Enable LeakSanitizer on 64-bit Darwin ASan llvm builds

Summary:
Also disables leak checking on lto tests, due to many leaks reported
in the system's ld64.

Reviewers: kcc, pcc, bogner, kubamracek

Subscribers: mehdi_amini, llvm-commits

Differential Revision: https://reviews.llvm.org/D37781

llvm-svn: 314535