[gen_ast_dump_json_test.py] Copy to binary directory to omit --clang argument

The script will now check if a clang binary exists in the same directory
and default to that instead of requiring a --clang argument. The script
is copied to the clang build directory using CMake configure_file() with
COPYONLY. This ensures that the version in the build directory is updated
any time the source version changes.

See https://reviews.llvm.org/D70119