[UBSan] Use shared library for UBSan on OS X (Clang part).

Summary:
UBSan is now used in the same way as ASan, and is supported on
OSX and on iOS simulator. At the moment ASan and UBSan can't be used
together due to PR21112, but I hope to resolve it soon by
embedding UBSan into ASan.

Test Plan: regression test suite.

Reviewers: zaks.anna, kubabrecka

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D8471

llvm-svn: 233035