[sanitizer] Max out allowed macOS deployment targets to 10.9

It's not a good idea to build the sanitizers with e.g. -DCMAKE_OSX_DEPLOYMENT_TARGET=10.12, because some deprecated functions that we intercept will cause build errors. Let's limit the allowed deployment targets to 10.9 (which is the default anyway), and warn when it's set above.

Differential Revision: https://reviews.llvm.org/D26557

llvm-svn: 286859