Minor fixes to scripts interfaces

* Remove all references to clang-tools-extra in helpers

We do not support clang tools extra in the python scripts, since it's
too complicated and nobody is working on it yet. Therefore, clean up all
references to it in the helpers as well, to avoid any confusion.

* Add a bit more info to llvm.py --help

Change-Id: I5990925cebafce182b081227a4d33aecead7b4ac
7 files changed