Add cmake build support for lsan on OS X

Summary:
Adds a new cmake flag 'COMPILER_RT_ENABLE_LSAN_OSX', which enables lsan
compilation and is turned off by default. Patches to fix build errors
when this flag is enabled will be uploaded soon.

This is part of an effort to port LSan to OS X, but LSan on OS X does not
currently work or pass tests currently.

Reviewers: kubamracek, kcc, glider, alekseyshl

Reviewed By: kubamracek

Subscribers: danalbert, srhines, mgorny, llvm-commits

Differential Revision: https://reviews.llvm.org/D29783

llvm-svn: 295012