Add a high level target for libc unit tests called 'libc_unittests'.

[sivachandra]

Summary

All other unittest suites will be dependencies of this target. This
gives us a clean way to run all the unit tests, and just the unit tests,
in a single step on a buildbot builder. When we add other kind of tests,
we can add a highlevel target for them as well.

[sivachandra]

Abandoning this change as we now have a target called check-libc.

libc/cmake/modules/LLVMLibCRules.cmake

function(add_libc_unittest target_name)
   )
   if(LIBC_UNITTEST_SUITE)
     add_dependencies(
       ${LIBC_UNITTEST_SUITE}
       ${target_name}
     )
   endif()
 endfunction(add_libc_unittest)
+
+add_custom_target(libc_unittests)
+
+function(add_libc_unittest_suite target_name)
+  add_custom_target(${target_name})
+  add_dependencies(libc_unittests ${target_name})
+endfunction(add_libc_unittest_suite)

libc/src/string/CMakeLists.txt

-add_custom_target(libc_string_unittests)
+add_libc_unittest_suite(libc_string_unittests)
 
 add_subdirectory(strcpy)
 add_subdirectory(strcat)