[sanitizer] Unify and generalize Apple platforms in CMake and lit test configs

There's more platforms than just "ios" and "iossim" that we should support, and adding more lit config variables for each platform isn't great. Let's generalize and have a single value that determines what the platform under test is.

Differential Revision: https://reviews.llvm.org/D48309

llvm-svn: 335123