Clean up the 'check' CMake build rule a bit, notable renaming it to

'check-llvm'.

Don't worry! 'check' still works! =] To rationalize the names of targets
used to run tests, the vague plan is the following:

make check-llvm  # run LLVM reg/unit tests  (currently 'check')
make check-clang # run Clang reg/unit tests (currently 'clang-test')
make check-rt    # run CompilerRT reg/unit tests
make check-asan  # run ASan reg/unit tests (subset of -rt)
make check-tsan  # run TSan reg/unit tests (subset of -rt)
make check-all   # run as much of the above as is available

The last one respects what projects are checked out and built for
a given tree. Personally, I would like to eventually make 'check' be an
alias for 'check-all'. For now however, it is an alias for 'check-llvm',
and thus no behavior has changed.

While this patch and my plan only really apply to CMake, I think it
might be good to similarly rationalize the naming scheme for the Make
builds.

llvm-svn: 159258