Retire CLANG_IS_CC option

A small reduction in build infrastructure complexity; when we had both
Clang and GCC in the tree it was useful to have both built, and choose
one or the other to install as /usr/bin/cc. Now only Clang is in the
tree, and there is no point in building and installing base Clang but
not providing it as cc (and c++, cpp).

Reviewed by: imp
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D37075