[CMake] Make llvm_ExternalProject always call the build action

This makes it so that when running 'ninja test-suite' from the top-level LLVM ninja build it *always* re-runs the ninja command in the test-suite directory.

This mechanism is required because the top-level ninja file doesn't have a view into the subdirectory dependency tree, so it can't know what, if anything, needs to be rebuilt.

llvm-svn: 265863