Switch Clang to use the new common LLVM CMake infrastructure for adding

lit testsuites. This sinks all management of the aggregate lit runs into
the LLVM CMake files, making Clang only responsible for declaring its
own testsuite. In the process we fix numerous "bugs" where the proper
method of invoking lit has changed over time, and the old system
encoded several broken artifacts of this in ABIs and compatibility
tests.

It also switches to 'check-clang' for the canonical name of the test
suite, although 'clang-test' remains as an alias.

The situation when Clang is being built in standalone mode is little
changed. It replicates just enough of the lit setup to cope with the
oddities of being run outside of an LLVM build.

llvm-svn: 159483