[Driver] Sanitizer support based on runtime library presence

The runtime libraries of sanitizers are built in compiler-rt, and Clang
can be built without compiler-rt, or compiler-rt can be configured to
only build certain sanitizers. The driver should provide reasonable
diagnostics and not a link-time error when a runtime library is missing.

This patch changes the driver for OS X to only support sanitizers of
which we can find the runtime libraries. The discussion for this patch
explains the rationale

Differential Revision: https://reviews.llvm.org/D15225

llvm-svn: 337635