It makes sense to just set this here since we aren't testing that functionality
and enables us to test this if we aren't using one of the specified Xcodes
(which can be useful).
This is meant to support scenarios in which we need to build as little as
LLVM as possible for performance reasons (e.g. when enabling LTO).
While LLVM CMake build system offers options in this sense,
in our investigation they turned out not to be suitable,
since either they are not granular enough (`LLVM_INCLUDE/BUILD` flags)
or they require active opt out for any new tool added
(`*_BUILD_*_TOOL` flags)
When using this mechanism, there is the possibility to specify different
targets to use for cross-compile hosts.
Supports rdar://32019390
