[lit] Only enable LSan on darwin when clang supports it

Summary:
LSan on darwin doesn't exist on older versions of clang,
causing non-boostrapped sanitized buildbots to fail

Reviewers: kubamracek, qcolombet, sqlbyme, zturner, modocache

Reviewed By: zturner

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D38703

llvm-svn: 315333