fix "can't remove clang warning #16" issue, remove -cc1 from clang command

read on http://clang.llvm.org/docs/FAQ.html

> clang -cc1 is the frontend, clang is the driver.

> Users should not run clang -cc1 directly, because -cc1 options are not
> guaranteed to be stable.
do not initialize plugin for non readable files

for example, fugitive is using temporary files like fugitive:///myfile.c for Gdiff, Gedit operations