Re-apply "test: Use add_lit_testsuites so that subsets of tests can be specified"

This version should actually remove the empty directories I removed
all of the files from. Thanks to tstellar for pointing out git-svn's
--rmdir flag.

Original message:

This creates make/ninja targets like check-clang-codegen and
check-clang-unit, much like LLVM already has. I had to move some input
files into Input directories so they weren't picked up as test
directories.

llvm-svn: 274565