[CMake] check-clang: Deprecate --path, according to r197576.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@197686 91177308-0d34-0410-b5e6-96231b3b80d8
[CMake] check-clang: Append items to CLANG_TEST_DEPS rather than set. CLANG_TEST_DEPS can be set in parent scope.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@197687 91177308-0d34-0410-b5e6-96231b3b80d8
[CMake] check-clang: Include ClangUnitTests if it can be built. Check existence of gtest.h then.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@197688 91177308-0d34-0410-b5e6-96231b3b80d8