Fix empty compile_commands.json

This commit re-enables the keep_last_cmd member variable in the TaskBase class (which was disabled by default in Waf 2.0.9) when clang_compilation_database.py is used. The clang_compilation_database.py file was correctly updated in the relevant Waf commit introducing the change, but the change is missing in ns-3's bundled version of Waf.

The issue and patch contained in this merge request were already posted on the bugzilla tracker as bug number 3009.