Bump sanitizer macOS deployment target to 10.10

Bump default value for `SANITIZER_MIN_OSX_VERSION` to 10.10 (from 10.9).
TSan does not work on macOS 10.9 and a nice error message is preferable
to an "unreferenced symbol" error when loading the TSan runtime.

We could try to only bump the deployment target for TSan, but we would
have to invest into adding support for this to our CMake build and it
does not seem worth it.  macOS 10.10 was released in 2014.

rdar://31335781