[CMake] Apply r279151 cleanup to libc++abi.

This patch applies changes similar to those in r279515 to libc++abi.

Summary of changes in this patch:

* Renamed variable LLVM_CONFIG -> LLVM_CONFIG_PATH
* Renamed variable LIBCXXABI_BUILT_STANDALONE -> LIBCXXABI_STANDALONE_BUILD
* Add an include of AddLLVM in the tests subdirectory for add_lit_testsuite.

llvm-svn: 279936